Set default volume at 50%

[cocarrig]

Now that volume can be set and more audio content will be adopted in DAKboard screens, a more reasonable volume level should be set for a baseline. 50% should be a good starting point and the user can set high or lower as desired.

[jatoric]

Completed and released in 4.02:
https://github.com/dakboard/Hardware-OS/releases/tag/4.02