Since AAC-support is pretty unusual, it would probably be advertised. I don't know if there even is such a thing as a aac capable AV reciever.
Your options are:
1) try to find a program that converts AAC to AC3 on the fly for Linux (not sure it exists)
2) use AC3
3) or connect your soundcard to your receiver with analog cables. You'll need a 3 3,5mm to 6 RCA cable (or 3 1 3,5 mm to 2 RCA cable) Should be availible in a RadioShack near you
