I dont know why you cant start without hdmi.
But if you use an hdmi cable the sound will indeed get sound over hdmi (would be nice if we could disable this)
Is there some command that can do this?
you need to add the following line to /boot/config.txt (I think the last one does the trick):
hdmi_ignore_cec_init=1
hdmi_ignore_cec=1
hdmi_force_hotplug=1