Personally i take any circuitbent signals through a mixer before going into a soundcard, but then i test everything through a small guitar amp before it goes anywhere near more expensive equipment.
You should be pretty safe anyway but if you need a calm a signal down a bit use this circuit:
R1 can be altered for a different output level. Use a higher value for R1 for a lower output at the jack.
Hmmm, you'd probably have to check out the specs of your soundcard for that one, or contact whoever made it as it may not be commonly available information.
I'd certainly be interested in a rough guide to that as well if anyone knows the answer.
On a related note theres smple circuits for interfacing just about any mic, speaker or line level signal with anything else you might come across in circuitbending here:
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