I gave up on ffmpeg and switched to sox_ng and now I've got it decoding, but is the quality perfect? would a slightly different set of options make it sound better?

I DON'T KNOW! I DON'T KNOW HOW TO KNOW

to make this better, I either need to listen to a bunch of WAVs with different ADPCM decoding options, OR learn the internals of the PS2's audio chip AND MIPS assembly.

To give you an idea of how much I hate audio, I am seriously considering the latter

I also can't compare two audio files! how can I tell which is better?

I listen to file 1, then I listen to file 2, but by the time I listen to file 2 I have forgotten file 1.

I can easily visually diff two images or show them side by side but I can't do that with audio

@foone For comparing the audio by listening, I usually use Foobar2000, load both tracks and right click > Utilities > ABX tracks... Really good for blind comparison, allows you to use Replay Gain and DSP if needed.

There's also Utilities > Bit-comparev tracks... That would do a full decode and bit-bit comparison (but not useful if you're comparing different compressions of the same track obviously)

@foone And visually I use Adobe Audition or Audacity;
in Audition once you load the tracks make sure you enable from the menu View > Show Spectral Frequency Display (or Shift+D), you can fine tune things visually from the additional options when you right click the scale in the right side. (not sure if it shows spectro in multi track mode though)

(sorry for the bad photos 💜)