TT-1100USB: why is the volume from my turntable so low?

Your TT-1100USB uses a magnetic pickup, so it does not put out a line-level signal the way a CD player does. The signal coming off the cartridge is much weaker and needs to be boosted before it will play at a normal volume.

To bring that signal up to line level, you need a phono preamplifier. Most DJ mixers have a phono preamp built in, so if you are running the turntable into a mixer, connect it to a phono input rather than a line input. If your mixer or receiver does not have a phono stage, you can buy a standalone phono preamp separately from most electronics retailers. Searching online for "phono preamp" is a good place to start.