The weak audio was not a level at all: the radio was still modulating from its
front microphone, so almost nothing of the USB feed reached the air. On an FTDX10
that is MENU → SSB MOD SOURCE = REAR.
Worth putting in the settings hint rather than leaving in a conversation — every
operator wiring a voice keyer for the first time meets it, and no amount of gain
in OpsLog can compensate for a rig listening to the wrong input.