[SoX-users] how to adjust synth mix gain level?

[forget c. <for...@gm...>]

I have a feeling this will involve multiple invocations of sox using a |,
but am not sure yet.

I want to be able to mix synth output with a pre-existing wav file. I could
do this as follows:

  play orig.wav trim 0 3 synth 0 brownnoise mix

Which appears to equally mix the noise with the original. However, I want
much less noise signal than that, only a hint of it. I can apply gain after
the synth, but that applies to the entire chain. How can I know the gain of
the noise down a lot but let the original come through much louder?

thanks
fg