I hope this helps someone, as this issue has been elusive and frustrating to me.

Issue: Periodically, WSJT-X will apparently fail to send audio, even though my IC-7300 is keyed into transmit mode. No audio, therefore no RF power is generated.

Result: Busted QSO's, causing retries of messages on both sides.

Setup: One USB cable to the rig, Windows 10, Icom USB audio driver, Ham Radio Deluxe selected in WSJT-X as rig control, split ops with Fake It.

Cause: It appears that sometimes WSJT-X and HRD Rig Control don't play nice together. There is some factor that causes them to not sync perfectly every time. It could be the rig polling rate of HRD, such that sometimes it keys the rig a bit too late, or it changes the VFO frequency while WSJT-X is attempting to initiate its FSK audio. It appears to be a resource contention issue, where both WSJT-X and HRD are trying to use the same USB channel for conflicting purposes.

Solution: Have WSJT-X control the rig, thus ensuring that there is not timing / resource conflicts between two programs attempting to signal over the same USB connection. It also might be solved by adding a CAT cable for rig control. I have a USB -> CAT cable on order and will test when it arrives.

Replicate the issue: At first it seemed random, at rates of 1-in-10 to 1-in-5 xmit cycles. But then I manually changed the VFO transmit frequency with a shift-click on the waterfall, just as my xmit timing hit. The rig was keyed with no audio. That's when I discovered that manual attempts to initiate rig-control during the xmit cycle can create the issue (I have frequency changing during xmit disabled in WSJT-X settings).

Other theories eliminated:

• RFI in the shack: Choke balun at the feedpoint, good shack grounding, and copiously applied RF chokes to all cables to and from the rig and the computer. Heavily shielded USB cable between the rig and the computer.
• Windows USB power-saving: Ensured all USB devices have power-saving disabled, and disabled globally it in Windows Settings.
• Audio driver issues: Ensured that every app in Windows, including the default, had other sound drivers selected in the advanced sound settings. Ensured that WSJT-X did not lose config of which audio driver it should be using. Ensured that Windows default wasn't getting set to use the Icom sound drivers. Did not see any evidence that the wrong USB audio device was actually receiving the FSK audio.
  
  

Still to discover: Does the problem still appear when I'm using a frequency that doesn't require a VFO shift of the xmit frequency? In other words, is it only the VFO action that causes the conflict?
Again, I hope this helps someone out there. I did a lot of Googling for this issue and failed to find something that addressed my particular issue.

- Craig, KN4VXX

"Me too"....  I have the same setup and problems. I will try not using HRD and see if it works for a while. 
Thanks for the information !

I am also experiencing intermittent transmit audio.  My setup is WSJT-X v2.1.0 and JTAlert v2.15.1, both running on a Win10 PC connected to my TS-590SG through the USB port.  When WSJT-X goes into Tx the rig correctly also switches to Tx mode by CAT control, but there appears to be no audio coming from the PC.  If I'm monitoring the sound out from WSJT-X using Control Panel/Sound when the problem occurs, I see no indication of audio out to the TS-590 USB CODEC.

This only happens occasionally, but always on only the first transmission when trying to initiate a contact.  The second transmission always works.  I have not been able to reproduce this problem on demand.

Any additional thoughts?

Thanks,
Dave, K8WDA