Synthesizers don’t just make sounds; they modify them too.  There’s no rule that says you can’t use one to modify or mix sounds that come from outside the synth.  There are hundreds of places to get electronic audio signals:

• Grab your iPod and plug it into your system.
• If you’ve got a computer running a soft synth, it’s a great way to expand your system.
• Patch in a weird-sounding toy with a headphone jack.
• Dig out your old CD, cassette, reel-to-reel or 8-track player.
• Hook up a microphone and be your own oscillator.
• Peel the shielding off of a patch cable you’re not particularly fond of and go hang out by a radio station.

It doesn’t matter; sound is sound.  Patch it in and use it.