In my PbP game, we just had a force 5 Mob Control spell land on a group of NPCs. Whilst still under its effect, another PC commanding voices one of those NPCs to lay down. His magic is 4. What would happen in this situation? The NPC is effectively under the control of the initial caster and his spell. Since the spell is force 5, it is technically more powerful than the magic 4 commanding voice. Anyone have thoughts on how this would work?

As the NPCs are not under their own control, the Commanding Voice would have no effect.

Roll-off, I think. I believe Commanding Voice requires a Magic + Skill roll to work, and targets roll willpower or strength or something to resist, yeah? Have the PC with Commanding Voice roll vs the Control Mob spell as if it was a resistance roll for the spell.

Alternately: nothing happens. Control Mob controls their bodies, not their minds; they can want to obey the person with commanding voice, but they are being puppeted.

Alternately x2: is the spellcaster who is using Control Mob within the area of affect of Commanding Voice? He might unconsciously order the mob to lay down as *he* does so.

Mob control allows the caster to "command" their targets to move using simple actions I believe. Maybe if the "command" is the same "command" as the commanding voice, it becomes a free action? Since caster barely has to exert any will to force the target to do the action?

If the Commanding Voice command contradicts the Mob Control, I'd just give the Control'd another save.

Yes.

That's all the discussion this topic generated? Slightly disappointed. I feel like there are far reaching implications. Multiple people attempting to control someone's mind could lead to interesting and unintended consequences.

Obviously, the topic sparks little interest, because when you're facing a mage mind-controlling a bunch of grunts and directing them to attack you, you don't mind-control the grunts back.
You geek the mage.

Eh, I don't think it's as far reaching an issue as you'd think. No more far reaching than Control Thoughts on a bound spirit who's been ordered to do something contradictory, or using Alter Memory on someone who's had false memories implanted with an ASIST black box.