Some cables cannot be used in EMT conduit and most building codes specify CL-2 or 3 rated cables for inwall use.
If you opt for conduit, I HIGHLY recommend NOT running any other signal in the same conduit with speaker lines. That is a headache for crosstalk and other audio nasties waiting to happen.
The same holds true for digital clocked cabling such as alarm lines and HA cabling.
For the vast majority of home stereo systems, 14 awg is quite sufficient and defintely NOT monster or other similar hyperbole laden cabling. Standard off the shelf in-wall rated cable will work just fine.
Actually the independent labs over many years have stated that Romex is just fine for speaker line.