Did you try more than one video?
I don't have any problems with sound in flash at all. WRAL,com (my local news station) is loaded with flash videos, go there and give it a try.
you can also try m.flash.com and make sure media volume is turned up in settings.
If the problem persist, try re-downloading the flash 10.1 beta.
After that go to m.flash.com .. then use the volume buttons on the side of the phone to turn up volume.
I have had no problems with the video or audio with the v3 of beta.