I occasionally find myself with no engine noise although ATC, flap operation sound etc are all there.
A recent malfunction was only affecting one aircraft - a DH Dash 6 "Twin Otter" and I noted that if I changed planes (via ALT+Enter) sound would be present on that plane and if I switched back sometimes the Twin Otter was OK.
I had a look in the "Sound" folder and examined the file which had a number of "Alias" settings - the one in use was "fsfsconv\prosnd22" and I could see no signs of this in FS9 "Sound". One of the non-activated "Alias" settings was for "Beech_King_Air-350\Sound" so I switched the colons. Sound was then present, and probably better too.
On the basis of what I found it looked to me as the only way of putting the "prosnd22" file back into use was a complete de-installation and re-installation of FS which seemed a bit pointless to me.
Have other folk experience this sort of "glitch"?
I have a similar one, when loading up FS sometimes there is no sound at all and I have to reboot my comp to get it... I think thats my comp though.