By modifying the exhaust - you can do this many ways.
EG: Buy a custom exhaust and fit it as a replacement. Drill holes through your catalytic converters porcelain. (refer to your bikes model and go read some forums that are just for your model - such as found on sportsbikes.net)
If it's a cruiser you could slip on just new mufflers to enhance your sound greatly.
Older bikes have packing in the mufflers to lower the sound, you can remove the packing to your taste.