Mobile phone control of network music server
I'm looking for a method to use my mobile phone (Galaxy S9) to control an audio stream (cast) to various point in my house, each of those points having a Chromecast Audio adapter. The Chromecast will provide analog audio to various amps I have built with guidance from people here on DIYAudio. Not really necessary to do different audio streams simultaneously, though that would be nice.
What I need is an Android app that will not only select what to play from my network music archive, but it must control the volume level (via the mobile phone). My music archive is mainly mp3 format, but I also have some flac files I would like to play when testing an amplifier/speakers. These files are all on a pc connected to the same network as the wifi on my phone.
I've gotten some apps (All Music, VLC) to work, but not with volume control, which is useless to me. I have a Plex server running on the same pc as the music archive, but can't find an App that will also control the volume as well as the selection of song.
Any suggestions would be appreciated. The simpler the better - I'm not ambitious/skilled enough to dig into my Raspberry to make it a headless music server... unless I absolutely have to.