So I think the reason you are getting an error is because Awake is called during the initialization of variables and so bgmClip[0] does not yet exist, and hence even though you are trying to check if it is null to see if it exists, it won’t be able to find any reference to it at all and won’t be able to work out if it therefore does or does not return null as it doesn’t exist!
Moving your awake stuff into Start should work, and possibly wrapping it in an if(bgmClip != null){ would help.
Just for variety here’s an example of how you might do it with a coroutine!