This one's basically a test/proof-of-concept audio mod for The Sims 3. I'm not sure if I'm legally entitled to do this, but given the fact that EA hasn't been hostile to anyone making derivative works of their Sims content, I decided to put it here.
Here's a video of the mod in action: http://www.youtube.com/watch?v=cgkI84WwLHc
The mod replaces one of the Buy Mode tunes with "Let's Go Shopping" from The Sims 1 - you know, that whimsical, Fifties-style tune that reminds you of that trip to IKEA or something.
This overrides the following resources in FullBuild1.package:
Sims3_Buy01 with type _AUD 0x01EEF63A and instance 0x1ACFD33E062C386C
Sims3_Buy01 with type _AUD 0x01A527DB and instance 0x1ACFD33E062C386C
Installing it is just like in most mods - just copy the included package files to your My Documents\Electronic Arts\The Sims 3\Mods\packages
AFAIK this one's the only mod out there that replaces the buy mode music with a custom track, so I'm not sure if there are other mods that conflict with it. But since this is an override it's best that you back up or do any precautionary measures before installing.
Jerry Martin for the original soundtrack, as well as Zenchreal for making this mod possible.