Why does converting a file twice make it sound worse?
Two very different operations called "converting"
Remuxing repackages the same audio and video streams into a different container — MKV to MP4, for example. It is fast and completely lossless, because nothing is re-encoded.
Transcoding decodes and re-encodes the content. That is what happens whenever the codec changes, and it always loses something. Confusing the two is why people expect a conversion to be free and are surprised when it is not.
Extracting audio from video
Pulling an MP3 or M4A out of a video is a transcode, and the file gets much smaller because the video track is discarded entirely. Quality is capped by the original: a 128 kbps source cannot be improved by exporting at 320 kbps — that only wastes space.
Match the source bitrate, or slightly exceed it, and stop there.
Which format to choose
For video, MP4 with H.264 plays on essentially everything; HEVC halves the file size but is less universally supported. For audio, M4A (AAC) beats MP3 at the same bitrate, while MP3 still wins on compatibility with older hardware.
If there is any chance you will edit or re-export later, keep the original. That decision costs storage now and saves quality permanently.
Our iOS app for exactly this. Runs on your device — no account, no tracking.