When I move to a new wifi my sound disappears

Every time I go to a new wifi login, even if it's my own at home, when I get on to Skype, my sound does not work and I have to reboot. An example is that I go to my aunt's house, whose wifi my mac recognises, I try to use Skype and get no sound so I have to restart the laptop to get it working, which is not ideal when I am already on a call. Then, when I go home again, the same thing happens. When I look in Settings for Sound it all looks like it's working fine. Any help appreciated.

Thanks!