The Free Lossless Audio Codec is supremely useful for archiving audio. You can compress audio using the flac tool, surprising though it may seem. This will compress it to the best size:
$ flac --best $SOURCE
But what if it can’t read the source? I’m trying really hard to avoid making one of my bad Tabasco puns I would have worked to death even a couple of years ago.
I previously batch-processed audio to aiff or wav first before feeding them in, before realising FFmpeg can transcode to flac in one step. And today I learned you can set an equivalent compression level:
$ ffmpeg -i $SOURCE -q:a flac -compression_level 12 $OUT.flac
I haven’t tested this on avconv, but I assume the same command would work.