The mono/stereo thing, explained
Mono means one channel. Stereo means two. That's it. Everything else is just what happens when you move between them.
Converting stereo to mono sums the left and right channels into one. If the left channel has a guitar and the right has a piano, the mono file has both at equal volume. If the left and right are in phase (as they usually are), the sum is clean. If they're out of phase (unlikely with normal recordings, but possible with certain stereo widening effects), you get partial cancellation and the result sounds thin.
Converting mono to stereo copies the single channel to both left and right. This does not create spatial information. The audio is identical on both sides. It sounds like mono played through two speakers, which is exactly what it is. The benefit is compatibility: some platforms reject true mono files, some players default to one speaker with mono, and some audio systems expect a stereo signal on the input.
Output is always MP3. The whole thing processes in your browser.
Who keeps running into this problem
Podcasters are the main audience. Most USB podcast mics record in mono. You plug in a Yeti or an ATR2100, record an episode, and the file has one channel. Upload it to a podcast host. Some hosts are fine with mono. Others reject it. Some accept it but the stats page shows mono as an error. Converting to stereo before upload just removes a variable you don't want to think about.
Music producers delivering to a venue PA system sometimes need mono. Many club systems sum to mono internally, but not all. If the venue says "send us a mono file," they mean it. Converting your stereo master to mono and checking for phase issues before you send it avoids the awkward moment where half your mix disappears on their system.
Someone digitizing old mono recordings (cassette tapes, vinyl, reel-to-reel) gets a file that plays from one speaker on most modern setups. Duplicating to stereo fixes that without changing the character of the recording.
Content creators compiling clips from different sources: some are stereo, some are mono, they're assembling them into a timeline and the editor keeps flagging the mono ones as mismatched. Batch-converting everything to stereo before editing standardizes the project.
Before you convert, know this
Stereo to mono is permanent. You can't recover the original stereo image from the mono file. If you think you might need separate channels later, keep the stereo original and only convert a copy.
Mono to stereo doesn't make audio louder, wider, or better. It just copies the signal. If your mono recording sounds quiet or flat, conversion won't fix that. You'd need a compressor or EQ for that.
The phase cancellation risk with stereo-to-mono is real but overblown online. Unless you applied a stereo widener plugin, used M/S processing aggressively, or the recording has a wiring problem, your stereo files will sum to mono without issues. If you want to check, sum to mono and listen. If something disappears, you have a phase problem.
This tool handles exactly two channel counts: 1 and 2. If you have a 5.1 surround file, this converter ignores channels 3 through 6. You need a surround downmixer for that.
FAQ
Should my podcast be in mono or stereo?
Mono, technically. Speech doesn't benefit from stereo separation and mono files are smaller. But if your host requires stereo, convert. Nobody will notice the difference on spoken word content. The file just weighs slightly more.
Why does my converted mono track sound quieter?
It shouldn't, significantly. The tool maintains levels during conversion. If you hear a real drop, your original stereo file might have extreme panning. Try normalizing the file first, then converting.
What's the difference between this and the channel remover?
The channel remover silences one channel but keeps the stereo structure. This converter either combines both channels into one (stereo to mono) or duplicates one channel to both sides (mono to stereo). Different tools for different needs.
Will this fix audio that only plays in one earbud?
If the source is stereo with audio on only one side, convert to mono. The audio gets summed to both speakers. If the source is already mono and only playing on one side, convert to stereo. The audio gets duplicated to both channels.
Can I go back to stereo after converting to mono?
You can convert the mono file back to stereo, but it'll be mono audio duplicated to both channels, not the original stereo image. The spatial information is gone. Keep the original stereo file if you might need it.