I've tried a few "experiments" to regain sound in MME Mode: Remove Creative Xfi system, Remove and Reinstall PTW 2010 (with build 4b), but the thing that appears to have solved the problem, is downloading and installing an older Vista 64 bit driver for my IDT integral audio system. This specific driver that Dell supplied with this PC (I reinstalled it when this problem began) seems to have a problem with PTW 2010.
I'm back in business now.
Thanks for your help,
Fla Musician