Do I have to open the HALdmc screen to play music or to control the music while it's playing?

No, you don't need to have the HALdmc screen visible to play or control music. You can load and play song lists, play specific songs, enable or disable random and continuous modes, mute, pause, stop, etc. - BY VOICE! The only situations where you need to have the HALdmc screen open are when you're creating or modifying song lists, modifying song information, scanning for music, copying music CDs, or adjusting the settings for the HALdmc.