This app was mentioned in 2 comments, with an average of 1.00 upvote
Three options.
Spotify: You can run Spotify (or any third-party spotify client that supports Spotify Connect) on your desktop and control its volume from the Spotify app on your phone.
MPD: Run Music-Playing-Daemon on your desktop, and control its volume using any mpd client on your phone, such as MALP on Android.
VLC: enable remote control on VLC and then control it using VLC-Remote on Android. https://play.google.com/store/apps/details?id=com.hobbyistsoftware.android.vlcremote_us
All my computers run Linux, but I can't imagine why these solutions wouldn't work on Windows too.
I used VLC Remote for a while before I switched to a Kodi machine.