Audio fixes and documentation updates

- Renamed `sound_id` parameters to `sound_name`, to avoid confusion with `instance_id`
- The `play` method now properly scopes its parameters to the specific instance ID that it starts (was applied to entire group)
- Simplify `onend` callback using Howler's builtin `once` method
- Added `instance_id` parameter to `mute` and `unmute` methods
- Updated documentation, fixing a missing return value, and defining the default behaviors when `instance_id` is not provided