I am trying to record to Audacity from my guitar and from a usb turntable and in both cases I am not able to adjust the recording volume using the microphone slider bar. The little circle on the slider bar is all the way to the right and I can’t move it. The speaker slider bar does allow me to adjust the volume . Any ideas what is going on here?
That’s normal with USB.
Good USB audio interfaces have a recording level knob, which adjusts the analog level before it’s digitized. If you are recording an electric guitar it also has a volume knob. 
If it’s too low you can boost with the Amplify effect after recording.
Low digital recording levels aren’t a problem unless they are WAY too low, or sometimes it’s an indication of an analog or acoustic problem. i.e. Turning down the knob on an interface while recording and leaving extra headroom doesn’t hurt the audio quality.
If it’s too high and you’re clipping the analog-to-digital converter inside the USB device, you need to lower the analog level, or live with it.
Thanks Doug. The audio interface and the guitar do have volume knobs so I can use those to adjust. The USB turntable does not have any way to adjust volume for recording and I was looking to turn it down since the recording volume was getting close to too loud.
I do appreciate your quick replies. Thanks again.
If your audio interface has line inputs, most USB turntables have line outputs. Sometimes there is a switch to select the line-output from the preamp or the direct cartridge “phono” output.
A guitar-instrument input will also usually work with line-level signals If it has stereo line/instrument inputs it should work.