How much softer? If its a small amount, normalizing the audio (Al Thompson's suggestion) would be perfect. If the audio is practically not visible in Audacity, and you normalize it you're going to get a whole lot of quantization noise in your output.

If you have flash volume up the whole way, ALSA up the whole way, and then record using JACK directly into Audacity, there is no reason for the recorded stuff to be any quieter.

I've used this approach before, and it works flawlessly, just check your volume levels. -Harry