This app was mentioned in 2 comments, with an average of 1.50 upvotes
If you don't need synchronous play then Plex/Emby is fine. Audio output can be done very easily with Pulseaudio piping over the network, or with ALSA if you have direct connection to the speakers.
If you want synchronous video play, Plex/Emby doesn't have this option. RARflix, a plex client for Roku allows you to begin watching a movie at the same timestamp as another user. This allows a kind of pseudo-synchronous play.
In this case, VLC is your best answer. It's multiplatform as well (here's an Android app). It can also transcode on the fly. You can play the stream locally on your server and send the audio out your central audio system. Other clients can simply mute their stream audio and listen to the central audio system while watching their video on their own devices. You might also be able to control the stream with a web interface as well... probably. No fancy stuff like plex but commandline VLC is an extremely powerful tool, which is useful for usecases more complex than simple single client streaming that plex/Emby provides.
Which "the remote app on android"? I'm using CJ VLC HD Remote (+ Stream) and it has a help screen that shows the settings for VLC.
WiFi or ethernet doesn't matter if the phone is connected to the same net as the PC with VLC.