RDPSND and RDPDR must be loaded together

Windows 2012 will silently fail to use RDPDR if RDPSND is not also loaded. Because Guacamole only loads RDPSND if audio support is enabled and present in the browser, explicitly disabling audio or using a browser which lacks full support (IE11) results in RDPDR failing.

Guacamole's RDP support will already load RDPDR if RDPSND is being loaded, but it should also load RDPSND if RDPDR is being loaded. If audio is explicitly disabled, or the browser lacks full audio support, RDPSND should simply ignore received audio packets.