As Glangford suggested, the easiest and cheapest solution is a Y-cable.
Sound quality is as good as the DAC/amp of the phone allows for.
If you want a wireless solution.
- a Bluetooth receiver at the McIntosh.
Just pair it and play ( as if you are using a BT headphone)
Bluetooth is lossy compression. Sound quality is as good as the DAC/amp of the BT receiver allows for.
You must stay in range (10 m)
This is Wifi hence a lossless transmission.
You might look for a Google Chromecast (Google has stopped the production of this piece of hardware ) or a streamer supporting Chromecast (the protocol)
Check if the apps you are using support casting.
This is the open standard for streaming AV over the network.
You need a streamer attached to the MCIntosh.
I’m not sure if Google play music on a phone can be rooted over DLNA to the streamer.
Might depend on the application (play to option)
Maybe there are streamers supporting Google Play directly.
Maybe this part of my website is of use: http://www.thewelltemperedcomputer.c...al_connect.htm