Not really, unfortunately. At least, you won’t be able to do it from your iPhone right away.
Using Remote to control your iTunes library and stream music to speakers via AirPlay is pretty neat. But, if a call comes into your phone while you’re doing this, you won’t immediately have a chance to pause or turn down your music. Instead, the iPhone screen will pop up the answer/ignore buttons.
If you answer the call, the music will keep playing, which may make it hard to hear your call. To turn down the volume or pause the music without ending the call, simply press the phone’s home button (you may want to put your caller on hold or speaker phone first – here’s to politeness!), go to Remote, and then pause the music or reduce the volume. This won’t interrupt your call and you’ll be able to resume streaming when the call is over.