MP3 Playback trouble

Just wondering if anyone can help. I've just bought a second hand psp and thought everything was working fine until I tried to transfer and play some mp3's. When the files are played they apear to be corrupted with a horrible pulsing clicking sound all through them. Also video files do a similair thing with clicking on the audio track and white squares all through the video. I've tried all sorts of mp3's and transfered from 2 different pc's and the problem is still there.
It seems to me as if the decoder is screwed, so what I want to know is if the decoder is hardware or software based and will a firmware update fix it?

Any ideas or suggestions are very much appreciated.

Edit- It's a v1.5 US machine