Blank screen with HDMI and AV plugged in

Using an Xbox 360 with a monitor. The HDMI works OK, until I plug in an AV plug, which I want to use for sound. At this point I get sound but a blank screen. It looks as if the AV plug cuts off the HDMI connection. Any solutions?

I don't think you can do what you are looking for. HDMI outputs both video and audio. When you attempt to use the AV Cable, you are effectively using a different output for both your video and audio. You can't have both at the same time.

The official Microsoft page shows the various ways to connect an Xbox 360 to a monitor or television. None of the ways listed show it using an HDMI for video, and AV Cables for audio. There does appear to be an "Xbox 360 VGA HD AV Cable" that you could use, which looks like this:

This would allow for what you are looking for, however your monitor would need to support VGA.

Depending on what model Xbox 360 you have, you could also use an optical cable for the audio, and HDMI for your video. But, you would need a device that supports optical inputs as well, such as a surround sound system. This would only be viable on an Xbox 360 S.