mu3-mods/Extras/SelectBGM/MU3/patch_Scene_32_PrePlayMusic_ChapterSelect.cs

using MU3.Game;
using MU3.Util;
using MU3.ViewData;

namespace MU3;

class patch_Scene_32_PrePlayMusic_ChapterSelect: Scene_32_PrePlayMusic_ChapterSelect {
    private ChapterSelectorItemViewData _selectItemViewData;
    private extern void orig_onChangeElement(int index, int indexRaw);
    private void onChangeElement(int index, int indexRaw) {
        var state = Singleton<Mod.State>.instance;

        // Dumb hack
        state.WithholdPlay = true;
        orig_onChangeElement(index, indexRaw);
        state.WithholdPlay = false;

        int selectorID = (
            _selectItemViewData.memoryChapterViewData?.memoryChapterData
        )?.getMemoryChapterSelectorID() ?? state.RecentBGM;
        Singleton<GameSound>.instance.gameBGM.playBGM(233, selectorID);
    }
}