[baldmosher]

Few likely culprits in order of likelihood:

1. the foam seals have perished and/or come loose in the ducting behind the dash - that's a dash out job and is very common as the foam Volvo used is rubbish

2. linkage to flaps motor has broken - again probably a dash out job (not sure) but you can gain access to some of the mechanisms without

3. cabin air filter is clogged and needs changing - least likely, but easiest job, needs to be done annually especially if you drive around town with lots of diesel fumes clogging things up

With those symptoms I'd reckon it's 1 - dash out is a 2-3hr job with someone who knows what they're doing or a 4-8hr job with someone who doesn't.