Can AirPort Express add speakers to a network without broadcasting a new wifi signal? I have a Time Capsule, which is 802.11ac capable. On wifi I get ~100mbps. 

I recently got some desktop speakers for my office. I like to move my laptop around the house, and I'd like to avoid having to plug the speakers in/out when I come into the office if possible. I'd also like my wife to be able to play music to the speakers, and her desk isn't close to them.

One option is the AirPort Express. I would use it in the office + plug my speakers into it, so I can just select the speakers as my output device when I come in. However, I wouldn't want my laptop to connect to the express's network, since it's only 802.11n, capable of ~50mbps.

Is it possible for the AirPort express to make my speakers available on the main wifi network, but for my laptop to somehow "ignore" the extended network coming from the Express? Can I choose to only connect to the "a/c part" of the network? Can the two spectrum bands be used to solve this?
Yep - just configure the express to "Join a network" instead of "extending" it.