Mob trigger isn't working on anything, the only time it seems to work is on the first ender dragon fight, but will not work when it respawns. I have also done tests on other vanilla mobs, with no luck

[enderdragon]
[enderdragon.trigger]
name = "mob"
priority = "15"
fade_in = "60"
resource_name = "minecraft:ender_dragon"
detection_range = "640"
fade_out = "640"

[voidworm]
[voidworm.trigger]
name = "mob"
priority = "15"
fade_in = "60"
resource_name = "alexsmobs:void_worm"
detection_range = "640"
fade_out = "60"

[mosco]
[mosco.trigger]
name = "mob"
priority = "15"
fade_in = "60"
resource_name = "alexsmobs:warped_mosco"
detection_range = "640"
fade_out = "60"

[wither]
[wither.trigger]
name = "mob"
priority = "15"
fade_in = "60"
resource_name = "minecraft:wither"
detection_range = "640"
fade_out = "60"

You need different identifiers for different mobs, otherwise you are just adding songs to the same trigger