Alright so I was looking around, and I found a way to convert songs, without using CDS.
Download the Codemaster's music converter:
Download
1) drag and drop your mp3 files into the program, and it'll attempt to convert it. Save your new tracks with appropriate names.

2) Rename your files from the .toc extension to .gta

3) Place them in psp/savedata/UCUS98603MUSIC


NOTE: If you get an error saying mp3 is not supported, try converting it to WAV first.

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