Fix game crash on Linux when some music files are missing

[fbraz3]

Description

Fixes #196

Changes

• Updated AudioManager::isMusicAlreadyLoaded() to search through all AT_Music events and return TRUE as soon as a valid, existing music file is found.
• Previously, it only checked the last iterated event in the hash map, which caused fatal startup errors on Linux if that specific file was missing or had case-sensitivity issues.