Although older Macs had limited options for audio output -- the built-in speaker or the speaker/headphone jack -- today's Macs offer a number of additional choices: USB audio, optical output (on G5-based Macs), PCI audio cards, and even FireWire audio. Unfortunately, one of the limitations of the Mac OS has traditionally been that you had to send all audio to the same output, which means that although you could play your games and movies through a set of external speakers to get great sound with bone-jarring bass, you'd also be stuck with startling iChat and email notifications that would at times scare you out of your seat. (Mac OS X 10.3 offers the ability to play system alerts through a different output than other sounds, but that's the extent of such customization.)
Rogue Amoeba's $12 Detour 1.5 (4 Mice) aims to solve this conundrum. Using Detour, you can route each application's sound to a different output; for example, you can send DVD Player's audio through your Mac's optical output to your 5.1 home theater system while restricting iChat's sounds to your Mac's internal speaker. You can also separately control the volume of each audio output.
Even if your only audio output is a set of speakers connected to your headphone/speaker jack, Detour can help by allowing you to control the volume level of each application independent of others. So you can blast your favorite game while keeping your email notification sounds subtle. No matter how many audio sources and outputs you have, you'll appreciate the ability to sort your sounds.