The Monoprice HDMI Audio Converter
The following from the switch description may explain the issue.
The converter supports all HDMI 3D video modes and high-definition, multichannel audio, including Dolby TrueHD™ and DTS-HD Master Audio™. A switch on the back of the converter allows you to switch between two different audio modes. Mode A allows the capabilities of the display to be sent to the source device. In most cases, this limits the audio output to 2-channel PCM. Mode B allows the source device to output any available audio mode. This allows you to select advanced multichannel audio streams for playback on your home theater audio system.