Why aren't I getting sound from calls?

[WPCentral Question]

No sound from calls....

If I select the speak phone setting, then I can hear the call. There is no noise and when I turn the phone up it doesn't work.

This seemed to have happened after receiving a call whilst using my Here Drive + but I can not set the call back.

Any advice without, telling me I've got to reboot as I've only just done this and it takes too long to get everything I need back on the device.