All, thank-you for the input!
This is is what I've found from a different forum:
"
General specs for input/output of Creative Soundcards:
...
> Line-Out:
>
> 0 to 2V RMS or peak to peak.
> Normal output level about 1V.
> Impedance assumed 10K Ohms, some cards 600 Ohms.
> Described as: Given a full Sine wave at 1Khz tone
> and a 10K Ohm > Load should produce about 1V RMS
> output.
"
**broken link removed**
I'm going to keep looking though, that source impedance seems kinda high........
I completely agree that the mic level will be significantly less than the MP3 player. Immediately downstream I was planning on having several op-amp circuits to gain and filter the signal, controlled by a micro.
I don't intend for it to normally operate with two sources at the same time, but rather am trying to avoid any chance of one of the sources damaging the other, or other components, etc. for example, WC both are tied together, and the circuit is in a loud room, the MP3 player set to max volume...
I do like the idea of keeping one op-amp solely for the mic, pulling the signal away from supply noise.
How's this second. drawing. If I understand correctly, I want Rpad to be >= then that of the Audio source/MP3's output impedance?
-EF