Audio Mixer Based on That of ao486

[spark2k06]

The audio system has been substantially improved as it is based on the same ao486, allowing volume control of PCSpeaker and Tandy Sound, as well as the addition of Audio Boost and Stereo Mix:

https://github.com/MiSTer-devel/PCXT_Mi ... prerelease

ao486_audio_mixer.jpg (184 KiB) Viewed 1028 times

Attached prerelease, for the moment we leave it here until we confirm that it works properly.

Thanks to @somhi and @Mills for all the testing you have done to improve this aspect of the core.