I have used SimpleControl for A/V control for many years. HomeKit itself, to the best of my knowledge, has no direct A/V control. SimpleControl (SC) has been outstanding - their newest version 5 does a lot more than their earlier versions. It can control HK devices too.
SC is a subscription based service - it uses the number of devices it controls to determine service levels and subscription costs. It controls your A/V system(s) via IP. Most devices today are controllable directly from this app. For those that are still IR controlled you can use one of many IR repeaters that are in-turn controlled via IP.
One thing to note - if you choose to also control HK devices - the app will count the total number of devices as part of the subscription price. For version 5 we currently do not bring in the HK devices - so our count is at the 30 devices level. If we bring in the HK devices we would go to a higher level. I believe the annual pricing is very fair for what this solution provides. Hope this helps.
I have a harmony with the hub and I use https://github.com/nfarina/homebridge with the harmonyhub plugin. It works pretty well, meaning that it can turn the harmony activities off and on. There is no control of volume or anything specific to the activity.However, with the harmony hub you can also use your phone or ipad as a remote through the hub (via wifi) which is pretty nice too and kind of fills in the missing controls.