This is normal. When started with ALSA backend, Ardour “reserves” all audio and no other application can have sound. If you need to listen to your music in some other player while Ardour is active, you must start it with JACK backend and use a music player that can play with JACK backend, too (audacious, mpv, mplayer, vlc, mocp - of all these, mocp will connect to JACK automatically…)