For a long time I used the 'what u hear' option when I recorded from Composer. It was necessary to make sure everything that might make a noise or cause a slow-down was closed.

However, I found that using the 'midi synth' option makes worry about system sounds, mail program downloading, etc., a thing of the past. The midi synth seems to operate in its own world and is unaffected by anything else going on.

Acoustica is the only recorder I have installed on my new (6 months ago) computer, but as I recall Audacity and Total Recorder also had similar options.

best,
mgj