Stuff that comes in handy when hacking:

Atrac3 codec.
Used for altering audio which PSP games play on the menu where you select them. This can also be used to alter entire soundtracks in some games (Puzzle Bobble Pocket). However most games use atrac3plus (Puyo Puyo Fever), which Sony is yet to release a codec for.
Heran Bago reccomends using this codec in conjuction with the Goldwave sound editor.

Chaotix ROM hacking guide
Something I worked on for a while.