Audio Repair and Restoration

Why a stereo recording goes thin or disappears in mono, and how to fix it in REAPER

By StudioRescue Editorial

Published November 1, 2026

You send the take or the episode to someone and they say the voice is barely there. On your headphones it sounds fine. On their phone, a smart speaker or a laptop with one speaker, the voice is quiet, phasey, or gone, sometimes leaving only the room noise behind. People hit this in every DAW: forum threads about it go back years in Audition, Audacity and older home-recording software, and it looks baffling until you know what is going on.

This is a different problem from a voice stuck on one side of a stereo file, where one channel is silent. Here both channels have the voice in them. They just disagree about which way is up.

Why one flipped side makes the voice vanish

A sound wave pushes and pulls. Polarity is which way the recorded wave goes on the push. If the left channel and right channel carry the same voice but one of them is upside down, then every push on one side is a pull on the other.

• In stereo each ear gets its own side, so you still hear a voice. It may sound a little wide or odd, but it is there.
• In mono the two sides are added into one signal. A normal copy plus an upside-down copy of the same sound adds up to almost nothing. The more identical the two sides are, the more completely the voice cancels.

Anything that is not on both sides in the same way, like room noise or a reverb tail, survives better, which is why the result can sound like a ghostly room with the voice missing.

Where a flipped side comes from

You will not always find out exactly why it happened, and you do not need to in order to fix the file. The common routes are:

• Wiring that disagrees. Modern balanced XLR gear puts the hot signal on pin 2. Some older gear used pin 3, and a cable or adapter wired the other way round flips polarity without making any obvious noise. If one of two inputs went through a cable like that, one side comes in upside down.
• A width or “stereo” effect. Some widening tricks work by putting an inverted copy on one side. They sound big on headphones and fall apart in mono.
• A file that was already like this when it reached you, from a recorder, an app, or an old project. You can still fix it.

Step 1: prove it before you change anything

Thin or hollow in mono can also be a timing problem between two mics, which has a different fix. These checks tell you which one you have.

Why the track polarity button does not fix it

Every REAPER track has a polarity (phase) button, and it is the right first move when two separate vocal tracks sound hollow together. On a single stereo item it does not help, because it flips the whole track, both sides at once. Both sides end up upside down together, they still disagree with each other in exactly the same way, and mono still cancels. The same goes for the invert phase option in Item Properties, which flips the whole take. You need to flip one side only, or stop using one side.

Fix A: one voice on both sides, keep one side

This is the usual case for a single mic: one voice, duplicated onto two channels, with one copy flipped. Each side on its own is a complete recording, so you only need one.

1. Open Item Properties for the item.
2. Set Channel mode to Mono (Left) or Mono (Right). Pick whichever side sounds cleaner if there is any difference. If they sound the same, either is fine.
3. Play it with the master in mono and in stereo. The voice should now sound the same in both and sit in the centre.
4. Check the level, since one side may play at a different level from the stereo version. Adjust the item volume if needed.

If several items on the track have the problem, select them all first and change the setting for all of them together.

Fix B: two genuinely different sides, flip one back

If the left and right are different signals, for example a real stereo mic pair or two mics on one source, throwing a side away loses something. Flip one side instead.

1. Open the track’s FX window and add the stock JS plugin that the REAPER user guide calls Channel Router w/ Polarity. Typing “polarity” into the FX browser search box is the quickest way to find it.
2. Make sure the input and output are both set to channels 1+2, then set Polarity Mode to Invert Left Channel Only. The user guide lists the other choices as Normal, Invert Right Channel Only and Invert Both Channels.
3. Switch the master to mono and listen. If the voice comes back full, you are done. Inverting the left or the right side gives the same mono result, so there is no need to try both. If neither brings the voice back, polarity was not the problem: set it back to Normal and look at timing instead.

If mono still sounds hollow after the flip, but no longer vanishes, there is a timing offset between the two mics as well. That is covered in why two vocal tracks sound hollow together.

Save the fixed version

Both fixes change playback inside REAPER, not the original file. If the file has to leave REAPER, render it. For a podcast or voiceover that will be published in mono, render in mono and listen to the rendered file once on a phone speaker before you upload it. The render steps are in how to export audio from REAPER.

What this will not do

• It will not fix a finished stereo mix where only some parts are flipped. If the vocal cancels but the music does not, flipping one side of the whole mix brings the vocal back and breaks everything else. Go back to the original project and fix the one track that is wrong.
• It will not fix hollowness from timing, where two mics heard the same voice a fraction of a millisecond apart. That needs the takes lined up, not a polarity flip.
• It will not make a width effect mono-safe. If the voice only falls apart after a widener, turn the effect down or remove it, as in how to make vocals wider in REAPER.
• It cannot recover a voice from a file that was already rendered to mono after the cancellation happened. The voice is gone from that file. Go back to the stereo original.

Stop it happening next time

• Record a mono mic on a mono input. One mic is one channel. Capturing it to a single input avoids the whole class of problem. The setup is in how to record vocals in REAPER step by step.
• Flip the master to mono once per session, and again before every export. It takes a second.
• Test new cables and adapters. If a new cable or adapter suddenly makes a two-mic setup sound thin, swap it out and listen again.