Unfortunately, if repair cannot solve the issue by replacing known good parts, you may need to try to get the attention of your local ASUS (technical) support to investigate it further (if they are separate from repair), but they also need to be able to duplicate this kind of issue on their end to escalate to others.
Anyway, if you haven't seen this thread about the G751 speaker crackling below, just wanted to run it by you too even though you probably do not have this series.
Not sure if it can help your situation, but sounds like some people were able to find some workarounds eventually although it doesn't sound like everybody was able to solve it.
https://rog.asus.com/forum/showthread.php?59248-G751-Sound-Crackling-Thread-(The-one-thread-to-rule-...