Compress audio in any of 23 formats: pick the format and a bitrate. The result is decoded again and measured against the original, so you see which frequencies were cut and can listen to both. Nothing is uploaded.
Where the file is going decides how hard to compress. Each card sets the format, bitrate, channels and sample rate above in one click, then measures the result on your audio.
Most providers cap attachments at 25 MB. Opus at 96 kbps fits a little over half an hour of audio under that limit; drop to 64 kbps mono and it holds close to an hour of speech.
MP3 at 128 kbps mono for speech. Every podcast platform accepts it, and a voice gains nothing from stereo or a higher rate.
Opus at 96 kbps is the smallest file every modern browser plays. Keep an MP3 fallback only if you must support very old browsers.
AAC at 192 kbps: small enough for thousands of tracks, close enough to the source that the difference is hard to hear.
Opus at 64 kbps mono. Speech carries little above 8 kHz, so the reduction costs nothing that matters.
Do not compress lossily at all. FLAC keeps every sample, and every lossy pass throws away detail you cannot get back.
Bitrate is simply how many kilobits each second of sound is allowed. Size grows in a straight line with it: double the bitrate, double the file. Uncompressed CD audio uses 1,411 kbps, everything below is a trade.
size (MB) = kbps × seconds ÷ 8 ÷ 1000
Three settings decide an audio file's size. Each has a point where cutting further stops saving space and starts costing quality.
Your ear can't hear a quiet tone sitting right next to a loud one in pitch, it's "masked". MP3, AAC and Opus measure this moment by moment and spend their bits only on what would be audible. That's why a 128 kbps file is 11× smaller than a CD yet sounds close to it.
Compressing twice breaks this: each pass throws away a different "inaudible" slice, and the losses add up. Always compress from the original, not from an MP3.
Trimming silence or switching formats can save more than any bitrate change. These tools pair well with compressing.