Deviation issues

Hello,

I am experimenting with BBP and a small FM transmitter (10Watt unit in a .5W mode into a 50ohm load) and I am monitoring it with my Service Monitor. What I am doing it playing random material from streams, downloaded FLACs, etc into BBP using the virtual pipelines. I have it working just fine into the application and am pumping the audio out the sound card and am taking the left channel and putting that into the line in on this FM TX'r. I can tune the levels on the computer and in BBP to be +/-75kHz, however when I change sources it can either exceed this horribly or have super low modulation. For instance, a web stream of a dance station is 75kc, then a talk stream is ~45 or 55kc on peaks. I would have expected that once I set the levels it would be the same regardless of the input, with only density changing. I have made sure to put my mod mon into peak and not average (my first thought of why I was seeing this) however after doing a sweep I noticed that higher freqs deviate more than lower freqs. My question to you guys, is this an issue with the sound card? I see no audio-enhancement crap enabled anywhere so I am wondering what I can do.

Note: The only other thing that has kinda worked is to run the audio interface on the PC at 100% and just let electrical limitations limit the deviation, however this does not sound nice at all. Ideas??