I found a good way to get audio, actually. Though it does have its drawbacks.
What you need is the PMF file,
PMFPlayer10, and
PSP Audio Recorder Beta. Don't use the 3.02 OE-B or 2.71 versions of PMFPlayer as Audio Recorder Beta does not like those.
I know IRShell has a PMF player integrated, but it doesn't boot correctly with Audio Recorder Beta.
put PMFPlayer10 in the right directory for Audio Recorder Beta's homebrew run, put the pmf in the root of your memory stick naming it movie.pmf.
Run PMFPlayer10 through Audio Recorder Beta, and Hit L+R+Circle as soon as you possibly can. Then hit Cross as close to the end of the video as you can, which pauses it, then hit L+R+Square to end the audio recording. If you wait until the video ends completely, PMFPlayer10 exits and Audio Recorder Beta doesn't get the chance to finish the file, and you get a 0 byte .wav. Wait for the memory stick to stop flashing, then hit Triangle to quit PMFPlayer10.
I used this to rip the audio from the japanese opening to Tales of the World: Radiant Mythology, so that I could then put together the english video (for the english title at the end) and the japanese audio (so the lyrics aren't butchered out). Then I inserted it into an ISO of the english version of Radiant Mythology. (which I legally own.) I got lucky though, because the music doesn't start immediately, and the music fades out long enough before the video ends to pause it and stop the recording.