PSP developer "plum" released a new version of his Mp3 Player Plugin for 5.00 M33, 5.03HEN, 5.50GEN, 6.20TN und 6.35 PRO!
It has now PSX-Support.
With this plugin you're able to hear your own songs while playing games.
Controls (translated by me):Pops should be made compatible
1. L + R +  = Mp3 Play / Pause
2. L + R + O = switch to playback mode
3. L + R + ? = next song
4. L + R + ? = previous song
5. L + R + Up / Down = volume +/-