Coming so soon after your other mysterious problem, it looks like some sort of instability. Either with MEP or your PC. Some obvious first steps you may have already tried:
1. Ensure this is not using anything whatsoever from the first project with the audio issue.
2. Reset MEP to default settings (noting first any customising you've already done and intend to restore).
3. Reboot your PC.
4. If necessary use System Restore to successive recent dates, checking the issue at each stage.
Thanks for your help Terry. I have decided to just buy a more recent version of this product. Probably wrong to reward product instability with a new sale, but there you are.