I've actually been looking into this. Basically, there's no easy way to rip the music, for now at least. In terms of extracting the ATRAC3 files from the .iso, it doesn't seem to be possible at the moment due to the encryption on it. I've spent hours seeing if anyone's developed a way to extract the data, but nothing really seems to be possible right now. Not even dumping the data via hex editing due to the encryption. Everything's packed into a single file, instead of being sorted out individually on the .iso itself, you see.
In other words, the only real way to do this, right now, seems to be by recording with a 3.5mm male to male TRS cable from the PSP to the computer. Which is precisely what I've started doing. If anyone can think of a better way, though, by God speak up. Because even though this isn't the best way, in terms of both quality and efficiency, it's better than nothing until they release an OST, eh?
Here's an example of a song I've ripped:
Jinouga Battle - FLAC (
http://www.mediafire.com/?hjf7km0y42ahwdp)
Jinouga Battle - MP3 (
http://www.mediafire.com/?p5wrp35d6t2r4br)
However, something that could speed up this process a bit is the help of anyone currently playing Portable 3rd. If you could help me think of what songs I need to rip, that would be great. Basically anything that hasn't been in a Monster Hunter game yet (in other words, no Tri stuff). If you want to hear those, you can just listen to the Tri OST!
You'll have to give me some time, as well. My friends and I still haven't completely beaten the game (currently at the dual Jinouga urgent), and one of them is out of town for a few more days. In other words, I don't have access to some monsters at the moment, such as Amatsumagatsuchi.
Any help with thinking up which songs to rip is greatly appreciated.