When a VRCSpatialAudioSource has "enable spatialization" checked, the first split second of the audio very often comes out bugged -- either too loud or too quiet. This is more consistent in desktop mode, but it also happens in VR. (It does NOT happen in the editor.)

I've made a world that demonstrates this here: https://vrchat.com/home/world/wrld_28350882-3cff-4bde-bb6c-47014de61b2a

A split second of missing audio might not seem significant, but the very start of a note usually characterizes its tone, and in the case of short 'notification blips' it can result in the sound missing almost entirely. (And padding the start with silence to work around the bug would result in the sound feeling 'late' compared to whatever visuals it's meant to accompany!)