Your question about amplification is a little unusual. I don't understand what you need...
On a purely technical level, an answer could be found in the
SGTL5000 datasheet on the
audio board page. If you look at page 2 of the PDF, you'll see there's an "MIC GAIN" up to 40 dB if you use the mic input, and another analog gain block up to 22.5 dB. There's another headphone amplifier with gain up to +12 dB.
Ultimately the amplifier can only output about a 3V peak-to-peak waveform (no matter how much gain), because everything runs on 3.3V. The headphone amp is able to drive ordinary 32 ohm headphones, but it doesn't have the current capability needed for 8 or 4 ohm speakers. If your amplifier question is about driving high power speakers, you'll need to add another power amp circuit to do that.
Of course, as you bring the signal through Teensy as 16 bit digital data, you could multiply the numbers using the
mixer object. But this is done digitally. Think "digital zoom" on a camera. Too much costs quality. So theoretically speaking, you could multiply by quite a large number. But I doubt that would be a really useful answer for you.
Really, I just don't understand why you're asking. So it's difficult to give you any sort of useful answer that speaks to what you're actually trying to accomplish. But hopefully these pointers to the technical specs will help you find the info you need?