[Examples] Qt warning on changing audio input in Audio Source example

Here is a Qt warning on changing audio input in standard Audio Source example - Examples/<Qt Version>/multimedia/audiosource/

QIODevice::write (AudioInfo): device not open

I suppose it doesn't depend on the platform or multimedia backend, but I reproduce it
stably at least on macOS 14.3.1, with the next steps:

1. Build and run Audio Source example
2. Try to switch audio input, e.g. from built-in "MacBook Pro Microphone" to any other available device (like "ZoomAudioDevice" or "AirPods") or vice-versa.

3. Observe the debug output.