Hypothetically speaking, let's say I've equipped a vehicle with an aftermarket audio system with an amplifier that has a higher draw (in amps) than my alternator produces causing my average voltage (while system is active) to drop to around 10-11 volts. How drastically can I expect this to effect the life of my alternator and battery?
If you put too much of a load on the alternator it will constantly max itself out as it tries to keep up with the demand. Running at max will heat up the alternator and eventually it will burn out. As @Brian pointed out, probably sooner than later.
Most aftermarket audio systems that require higher amps also have high output alternators.
This depends on how you use the audio system. First, make sure not to confuse peak power ratings with the average power draw. Music consists of a complex signal with a volume that continuously varies. Even when played at full volume, the amplifier will only draw full power during the highest volume peaks, typically only a few % of the total playing time. And unless you really want to damage your hearing, you won't ever play at full volume.
The average power draw of a custom sound system is usually well within the range that can be supplied by the alternator. The battery can supply the peaks.
The exception is decibel contests. In those, the system will play a sine wave at full volume for extended periods, and you may need a heavier alternator and/or more batteries.
Rule of thumb: if your amplifier is rated at 250 A or more (wiring is 50 mm2 or more), it's time to start thinking about your power supply.