I did the jailbreakme a couple days after it came out. I have an iphone4 with ios4.01. when I go to facebook videos that I have posted (videos taken from my phone) and try to play them, I get no audio. the onscreen volume control is there for a split second and then disappears. When I play them from a web browser on my computer they definitely have sound. You tube app has sound.
I have the following hack apps installed on the iphone in the even that somebody knows they might be an issue:
my3g
mywi4.0
torrentula
yxflash
ua faker
privacy
pdf loading warning
I updated the mobilesubstrate recently to fix the google maps issue (it wasn't showing traffic).
and probably a bunch of system stuff that gets installed without my directly knowing about it.