what's the maximum output voltage of my soundcard?
First: DAC or ADC ? DAC makes no senses in your question here.

Second: you could always protect the input of the things you want to plug into your soundcard.
Basic over voltage protection is easy achieve with a zener diode and a resistor.

Third: In fact input protection is compulsory here. You are talking about hot plugging something. Which means manipulation of the connectors, connecting things that are not at the same potential during the plugging etc. This is where a ESD discharges from the user or between the components may destroy your device. It's not a problem for the sound card which should be correctly protected otherwise it wouldn't have passed the norms.