There's a step by step Tutorial for this in the FAQ
Q: How can I delete samples stored in the Internal Memory (like stock sounds or whatever)?A: Well, actually you really can't just delete them. But what you can do is, overwrite them. At least in the AKAI OS or JJ-OS 1. In OS 2, the internal memory doesn't seem to be accessible any longer (at least in version 1.18). So, what you need to do is this:
- Turn Off Auto-Load for Internal Memory.
- Restart the MPC.
- Record some silence. 1 second is enough.
- Enter Trim Edit and change it's length from whatever it is to 'End: 00000001', so that you just have the smallest possible sample length. Press Edit [F6] and discard the rest.
- Assign that Sample in Program Edit
- Now, go to Save Menu and select 'Save: ALL PROGRAMS' and select the program you put that super small sample in - it's called 'Program01' by default.
- Press [Do It]. The following screen says: "This will erase all Programs and Samples in the internal flash memory". Press [Do It]. Done. At least it should be by now, but...
- If you're internal flash memory was full before, you probably can't save that program with the sample. It needs 10 KB of free memory at least to be able to save.
- If it didn't work, you have to load a program from the Internal Memory, with samples.
- Go to trim, and change the length of the samples just as in step 4) and discard the rest.
- Enter Save Menu again and save that exact Program, overwriting the old one. This should gain you the amount of free memory needed to save the other program.
- Now repeat Steps 6 and 7 and all the Internal Memory should be garbage free by now!