How can I create an air tight seal around pipes?
Solution step 1: Just seal all of the penetrations you can find. If that doesn't help, you'll probably have to start tearing off drywall to get at hidden pipes if you're sure the smell is seeping through gaps between plumbing and the floor.

Solution step 2: Use Spray-applied polyurethane foam. Dow Great Stuff works well. Just be sure to wear gloves!