The usual suggestion is to listen first to the same low volume vs. high volume voices using headphones. This would isolate if the problem is "inside" Motif or "outside". Assuming your headphones are working for both L and R channels and the PHONES connector is making good contact (both channels) with your headphones. Usually this isn't the first to fail since headphones tend not to be used as much as other connectors. If your headphones sound fine - proper volumes for everything - then the problem is "outside" Motif.

Assuming the headphones sound fine, further debug of your speaker output (no longer using headphones) ...

Try panning to see if that helps. If it does and assuming stereo output - then you have an issue with a channel. Might be the XF - but also may be cabling, devices/things between (mixer, DI box, adapters, etc) or speaker (connection, volume setting, blown, position, etc).

That's where I'd start.

BTW: there's also the usual question about the source of these voices with low volume. Are they all factory presets, user voices, or from sound libraries from external sources? If the issue is all with factory presets then this would be "better" - since we can trust unedited factory presets. Other content is less trustworthy in terms of programming - so it would be great if no doubt is entered into the equation due to the source of the problematic content.