Bugzilla

# Step to reproduce
0. Make sure there are at least two input devices in the system
1. Make a WebRTC call on https://appear.in/ and choose the system default input device as the mic
2. Use another device to join the WebRTC call
3. Unplug the mic

# Expect
The mic is switched to another input device

# Actual
No mic is active. And there is a warning message shows up:
> Looks like you unplugged your microphone.
> To allow others to hear you, plug in again or select a different input source.

## Note
- If the mic choosed in the call is non-default device, then the mic will be switched to the default input device automatically
- In Windows, the mic will be switched automatically no matter the mic is default or not.

# Step to reproduce
0. Make sure there are at least two input devices in the system
1. Make a WebRTC call via https://appear.in/ and choose the system default input device as the mic
2. Use another device to join the WebRTC call
3. Unplug the mic

# Expect
The mic is switched to another input device

# Actual
No mic is active, and there is a warning message:
> Looks like you unplugged your microphone.
> To allow others to hear you, plugin again or select a different input source.

## Note
- If the mic choose in the call is a non-default device, then the mic will be switched to the default input device automatically
- In Windows, the mic will be switched automatically no matter the mic is default or not.