To put comments to your Casablanca, you may use a microphone which only provides one channel. This will result in a weak and single-sided playback with a stereo project.

Connect your microphone (through a mixer or amplifier) to the left (!) audio input of Casablanca. Change the project settings to "VCR (Mono)". Only the left channel will be used as an input then. During playback, both channels will be used. Do not forget to switch back to "HiFi (Stereo)" later.

If you are using the audio quality "CD (Stereo)", there is no way to switch to mono on the fly. So you would hear your comment from the left channel only. After recording, use the audio special effect "Copy" (Settings: "Left --> Right"). This will copy the left to the right channel and you will get a high quality comment. You may delete the old sample now. Do not use the effect "Mono" for this matter. It mixes both channels which would reduce the volume of your comment.