Why can I hear my recorded signal only on one speaker?

When you are recording a mono source with only one channel through one input, such as the signal from a microphone or a guitar for example, you need to make sure to record on a mono track rather than a stereo track in your audio software (DAW). Otherwise you would have it only one one channel (either left or right), which you will notice when you playback the signal.

Your source is mono, as such it cannot be recorded to more than one channel. Because of that, basically every DAW provides the option to record on a mono track, which should actually be the default. Of course your DAW will output the signal through both speakers (controlled by the settings inside the mixer of the DAW) then.

The only time you would record on a stereo track is when your actual source is stereo with two separate channels (for example if you record the output of a CD player or turntable).

Please consult the documentation of your DAW to check how to setup recording on a mono or stereo track.