Re: Looking for 2 topics in the Tn...
Not sure exactly what you mean by 'filter midi by channel'. If you want incoming midi information to be assigned to channels discretely, go into your Project Settings, under Recording, and check 'Auto-demix by channel if multitrack recording'. Then just assign each channel strip to the desired midi channel, corresponding to the incoming midi channels.
As far as 'replacing audio' - not sure about that one. If you just want to increase the volume of the audio because it was recorded too low, you can select the region, and then go to the sample editor and under the Functions menu, select 'change gain' and increase the level as desired.
Hope that helps. Let me know if I misunderstood.