October (n9f27m) update still doesn't fix it. Two more months and it's a whole year.
Thanks for reporting this, the October (N9F27M) update is the last update the Nexus 9 will ever get. Very sad to see that Google just ignores this issue after messing up the sound of the external speakers almost a year ago.
I already gave up hope months ago and reluctantly performed a downgrade to 7.0. Now the sound is back how it supposed to be and the volume is adequate again.
Though the overall performance of the tablet is lacking. My OnePlus 3 is running circles around it.
In the beginning I thought quite highly of Google, a great search engine without ads and this opinion held up more or less over the years, till they killed the Nexus line.