I think I understand your volume problem because I had it too. When my SM10 was hooked up to my bike and then I connected a garmin there was enough inline noise that it caused the audio level to play low as if the SM10 had lowered it to hear the garmin.
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Yes! Noise on the SM10 aux input will suppress the main input volume. I ran into a similar problem on a ride with a guy who had a noisy mike (non-Sena). Any time I would put my intercom on it would suppress the signal I was getting from my SM10. That was an issue inside the 20S, though.
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