A rights holder provided a piece of music to use as background for the narration. However, no matter what I do, I can’t pass the ACX check for noise floor. I can only seem to get the piece to around -29db for the floor. I am using the Noise Gate plug-in set at -60db. Should I be using something else. I have never come across this problem before. Help please.
…I’ve never made an audiobook.
Is the background noise really THAT BAD bad on the music? Even if the music has a relatively-high noise level, if it’s truly background music it will be turned-down and the noise will be turned-down too.
Is that your measurement with ACX Check, or is that what ACX told you?
Do you have the required silence (“room tone”) at the beginning and end?
It might just be a quirk of the measurement where it’s hearing the music as noise, if there’s low-level music with no narration on top of it. You might have to remove any music where there is no speaking. Or, maybe turn it up when there is no music so it’s not analyzed as noise.
I am using the Noise Gate plug-in set at -60db.
Which noise gate? Is that the threshold? With a -60db threshold, the gate will be open to -29dB of noise. 
And if you set the threshold to -29dB (or -28dB) it’s going to sound terrible.
You’d have to “be careful” with a noise gate. It can sound unnatural and if your background drops to negative infinity ACX will probably reject you because the background noise cuts in-and-out in an unnatural way. The [u]Audiobook mastering wiki[/u] recommends mild noise reduction rather than a noise gate.
Apparently audible don’t approve …
https://audible-acx.custhelp.com/app/answers/detail/a_id/6673/~/can-i-add-music-to-my-audiobook%3F