...in my opinion is to buy a decent (and small) bluetooth headset and get that sound pumped straight into the ear canal.
Most of the reviews I read in the Bluetooth forum all mentioned there was a drop in volume using a BT headset but I contacted a fellow forum member (can't remember who but do a search on HeadOffice and headset and you should find it).
He recommended the Plantronics Voyager 510 Bluetooth Headset and I went for it. He also recommended going to www.cellphonemall.net
and phoning them (as the headset doesn't appear on-line). Enter extension 221 and ask for John - tell him some forum members recommended it and said he'd give a good price. I got it for $70 including shipping.
Works like a charm - is small and most importantly
has enough volume. I still run it on full volume but it's a lot better than just using the BB.
Interestingly - in the U.K. earlier this month my BB roaming onto the Orange and Vodafone networks had plenty of volume - no headset required.