Justin Sharrad is working on a FLAC player for PSP. FLAC is a high quality audio format, refer to a Wikipedia article if you want to know more about it.

In this early stage, it just plays the files as you select them with your up and down buttons. The X button starts playing and the O button stops playback.

You can leave feedback here: http://forums.qj.net/f-psp-development-forum-11/t-release-flacplay-early-beta-124628.html