I registered to post this fix -- this forum has been the most active about the issue of "no sound" on Windows 7, and while it didn't solve my issue it educated me to the widespread nature of the problem and kept me focused. This solution may also work for other than the ASUS PB5 series motherboards, and even for more than SoundMAX, but I can't test that myself. What I can say is that the solution, after hours of frustration, turned out to be so painfully simple I wanted to be sure I shared it.
My situation was that installing the Windows 7 RC yesterday resulted in the sound driver for my onboard audio claiming to all be standard MS issue HD Audio drivers, and basically dysfunctioning without any errors. Everything claimed to work, but there was simply no sound. I tinkered into the night, did a lot of reading, and got nowhere.
This morning I decided the lack of errors meant something, and went to the BIOS. This ASUS board has an entry in its "Advanced" page called "Front Panel Support Type" that I have been vaguely aware of the entire time, but never had to change for any other O/S I dual-booted for testing. Even original Windows 7 betas worked fine. It was set to [AC97] and the primary stable O/S (XP SP3) was still, and always did, work fine. The other option was [HD Audio]. I decided to give it a whirl and set it to that.
Booting to the partition with XP SP3 after that resulted in the device manager warning the device "SoundMAX Integrated Digital Audio Controller" was inoperable. I removed it and reinstalled using the 22.214.171.12440 build of the XP drivers. It worked like a charm.
I booted to Windows 7 RC and went to its device manager, which as usual showed no troubles. I removed the "High Definition Audio Device" entry that was disguising the SoundMAX. I scanned for changes, it correctly identified the onboard device, and it barked at me for a driver. Whether you use the XP version (126.96.36.19940) or the Vista version (188.8.131.52.w) it works. The XP driver, for me, works better and sounds slightly better. (I suspect their Vista version jumped through some hoop to make [AC97] work.)
I checked this by installing again from scratch, and as long as that setting in the BIOS was [HD Audio] Windows 7 correctly chose the device, though in my case it barked for the driver. Again, pointing it to the XP driver, sound was fine.
Again, I have no idea if this would work for other onboard sound devices (and obviously the issue is different for cards), but it is worth a try for anyone who has the issue. Just check your BIOS and see if you can shift from [AC97] to [HD Audio] mode. For whatever reason, Windows 7 seems to prefer the latter.
Good luck to all.