Why do I only hear the signal I’m recording in the left (or right) speaker?
If you are recording a single source, such as one microphone, a guitar, or a keyboard with a mono output, you will want to record to a mono track; recording a single source to a stereo track will result in the sound being placed only on one side of the stereo field.
In your DAW software, you'll want to either create a new mono track, or change the channel mode of your current track to mono, rather than stereo.
The only time you should record in stereo is when your source is stereo (i.e., you have two separate inputs -- one for left and one for right as in a CD player or keyboard.)
Below is shown the visual difference between a stereo and mono track in Studio One 2:
You would want to make sure the "channel mode" is set to mono for your mono vocal recording, as in the first track in the example above. The "channel mode" button is the button directly below the red "record arm" button and blue "monitor" button. A single circle represents mono, while double-circles represents stereo.