Volume setting not persisting after loading saves

## State your operating system:
Linux, gtk3

## DesMuME version
0.9.13

## Issue

When I load a save, volume is reset to default value.

## Reproduce

1. Open Config > Set audio volume > Change volume to any other value
2. Save state (e.g. Shift+F1)
3. Load state, volume is reset to default value now