Apparently the old sound bug from ArmA is still there in Arma 2 (or maybe it's a feature?). How are you playing the sound? Probably via "say", right, since the driver is talking? Well, the fun thing with "say" is that, when in a vehicle, it's not the units "saying" things, but the truck itself
(you can probably test this by going into 3rd person/standing outside the truck - it'll be chatting happily to itself - but anyone INSIDE it will hear it muffled).
I'm not sure if it's going to help, but use playSound instead (if you're being advanced, you can make a silent sound file with attached lipsynch created on the actual sound file, but few people actually bother to make the .lip files so...). Hopefully that will play it as loud as necessary.
However, since this problem does -not- occur in BIS missions, I'm going to have to assume it's something we're doing wrong. Maybe if you use BIS's own conversation system (
BIF link) that'd solve it, somehow? But it's hardly worth it for just one line of dialogue, and I'm myself not sure how complicated it actually is.
Oh well. Good luck!
Wolfrug out.