This one is both a bug and a feature request in that is seems Soundbox doesn’t know how to playback mono samples.
When loading in mono samples, they default to left channel only and if you try panning to the right to “center” the sound it just fades the left signal until there is silence so there is no way to center a mono sample.
Having a “Mono” button to declare sounds in a group as a mono source would fix this issue and either duplicate the left channel to right to make it stereo or else just center the sample. This could potentially be automatic as the Waveform reader within Soundbox is displaying the sample as a mono waveform, there just seems to be a disconnect with playback not being able to playback mono sound centered.
The only current workaround is to convert the sample to stereo by loading it into another sampler in my DAW and re-exporting it which is time consuming and not something I’m going to do for all of my previous samples recorded in this way.