Nah, its happened when playing career also (i think) but its more annoying when it happens online! Is it a known problem, if so is there going to be a fix? The only other thing i can think of is that my HDMI lead is loose. Will wiggle it later.
strange if sometimes you have sound and sometimes not, so hdmi connect looks good
do you use rapture or software on the menu ? did you tried the 2 options ?
I've had some audio disappear from me during the career mode where i can hear the gears shifting and tires on the ground well but the engine noise was barely audible. On the other side i've had one online game where i got what sounded like static through the whole thing (just muted the tv and dealt w/ it) both of these problems resolved themselves once the game reloaded after that session. This was ps3 not xbox but may point toward a bug w/ the game itself rather then your system
Yeh your right, have had simular things and someone i was racing the other day had audio failure. Defo had the static sound appear on Xbox and its even more noisy when you hit the rumble strips