It turns out that Teamspeak has a "feature" which means that as default it will auto-mute a channel if there are 100 or more clients connected. This can cause problems and if you do not want this to happen then the option to disable this is easy to change but it is hidden away:
- Right click the server node at the top of the channel tree
- Select "Edit Virtual Server"
- On the new popup, select the Misc tab
- Near the bottom is an option labelled "Min clients in channel before silence" which is set to 100 as default
- Change it to a larger value that suits your needs and click OK
- The change should take effect immediately with no need to restart the server