[BUG] A large set of sounds are not set to Mono, wasting space for builds

[jverral]

A large set of the default MRTK sounds are not set to Mono, so they will take up additional space in builds and take longer to load.

[whitetigerhelix]

There's basically no reason for UI sounds to be stereo in a 3d context, so it makes sense to use mono.

However, looking at the audio clips in the repo, most are actual mono audio files (not forced to mono), except for these three (gem, notification, voice confirmation). The first two of these look like the channels aren't the same based on eyeballing the waveforms.

The notification ones should likely be forced to mono.