What is the typical max voltage for sound card line in?
Nominal line level for consumer equipment is -10dBV, which is about

$$10^{-10\:\mathrm V/20} = 316\:\mathrm{mV_{(RMS)}}$$

or (for simple sinusoidal waves):

$$\sqrt{2} \cdot 316\:\mathrm{mV_{(RMS)}}
 = 447\:\mathrm{mV_{(peak)}}$$

or:

$$2\sqrt{2} \cdot 316\:\mathrm{mV_{(RMS)}}
 = 894\:\mathrm{mV_\text{(peak-to-peak)}}$$

Equipment will often include as much as 20dB of headroom above this before clipping, so a full scale signal could be 10dBV, or 3.16V(RMS), or 8.9V(peak-to-peak).

Of course, no manufacturer wants to be quieter than everyone else, so devices tend to have hotter outputs than they should, and inputs have less headroom than they should. Bend these rules to the extent that you think your customers care more about volume than fidelity.