Hm, I had a simmilar problem last night.
I wanted to empty my rechargeable batteries, so after saving everything I let a sequence playing until the batteries were empty and the MPC500 turned itself off. This morning I turned on the MPC500 (w/ the ACDC-adapter), loaded programmes and all sequences & songs, but the sequences won't play sound. After trying to load other programmes and sequences I realized that this problem affects all sequences I had saved on my CF-card. Triggering the sounds of the programmes does still work fine, and also the events within the sequences are still there - I checked this with STEP EDIT; they just don't play any sounds

Any idea anyone?
Thanks!