Fix PC sound effect playing twice and Trainers eye BGMs

[Aelysya]

This MR fixes three issues related to the recent update of the sound design config.

The PC events were playing the sound effects when openning and closing it, since these sound effects were directly added to the start_pc method, they were played twice.

The trainers meeting sequence BGM were also set in the events with the audio/bgm prefix, leading to the code trying to look for files like Audio/BGM/audio/bgm/file.ogg

Some of the battle arena's battles were calling to audio files with the audio/bgm prefix, leading to the same issue as above.

Tests to perform

• Try using every different PC events in the demo, their openning and closing sound effects should only play once
• Try being spotted by all the trainers in the demo, their eye_bgm should be played correctly
• In the battle arena, all the battles should correctly play their BGM (Especially the fights number 2, 7 and the one against Palbolsky)