Apple Mail no longer makes the "woosh" sound when sending mail

OS X 10.7.3, MBP

The other sounds seem to work in Mail. I have the sounds-on options checked (having turned them off and on again). It worked yesterday as far as I can remember. I have restarted the MBP three times since yesterday. I had a "finder not responding" problem on shut-down last night, which I resolved by killing Finder in Activity Monitor. I'm wondering if something has been corrupted. I have the 10.7.3 combo update downloaded (and installed), and my inclination is to re-install it, but I wanted to see if anyone knows a simpler solution. I do have regular Time Machine backaups available. Perhaps the Mail plist file would be worth restoring? Advice appreciated