Johnnyandrew, welcome to the Seven Forums.
I would start with re-installing/updating the drivers
for the Dynadock. Go here, Toshiba Support - Homepage
select laptop accessories, dynadock, and then the model you have (5 to choose from).
Once that is done then check in the Windows playback properties (right click the speaker icon in the right corner) and select Playback devices. Make sure that the device is showing there and if not, set it as the default playback device.