Thank you for the guidance.
There may be guests who would not have the phone app.
Regardless, it sounds like an in-line volume control is not recommended.
For guests who don't have the app, iPhone users can stream to the AVR using AirPlay- best way is to create a guest network for them to log on.
They work fine, but you would need to observe the power rating. Also, buy the kind that have a rotary control and make sure the desdcription mentions 'conformal coating' on the circuit board. I would use the type that DOESN'T have a bunch of resistors.
DO NOT mount it in an exterior wall of you want it to have a long, happy life- moisture can enter and cause problems.
Use speaker cable that's rated for direct burial, not zip cord with PVC insulation.
Are you using more than one pair of speakers outside? If so, this is a decent way to control each. Even If you need to adjust the relative level of multiple pairs, why would you not just use AirPlay or the Marantz/HEOS apps? Both apps allow control over Zone 2 and Zone 3. You'll need four additional channels of amplification if you want this to sound best. Don't skimp, but don't go all "I gotta have the best amp"- it's outdoors.
What speakers are you planning to use and how will they be arranged? Don't fire them outward from the house if you want happy neighbors- aim them toward the house whenever possible. It will prevent the need for extremely high power and the building will reinforce the sound.
If you want the speakers to be relatively hidden, one of the many 'landscape speaker systems' can work well- they have speakers on stakes that are similar to landscape lights and if you want bass, subwoofers are available.
