* The DVI-to-HDMI cable is not properly connected. Make sure that the DVI connector is plugged securely into the DVI port on your computer and the HDMI connector is plugged securely into the HDMI port on the receiver.
* The DVI port on your computer does not support audio. Some DVI ports only support video signals and not audio signals. Check the documentation for your computer to see if the DVI port supports audio.
* The receiver is not configured correctly. Make sure that the receiver is set to input audio from the RCA jacks. Check the documentation for the receiver to see how to do this.
If you have tried all of these things and you are still not getting receiver sound when there is an HDMI signal, then you may need to contact the manufacturer of the computer or the receiver for help.