Miguel, thanks for the tip on using Voicemeeter to fix the USB volume.
For some reason the VARA-HF modem software does not put full-scale audio into the QMX USB sound device (at least in Windows).
Some posters have recently mentioned using a 'speakerphone' setting in the VARA soundcard settings to boost the volume, but my VARA-HF doesn't seem to have such a setting, and I have the latest.
