There are a couple of spells which explicitly work only when the target "can hear you":
- Suggestion — "a creature you can see within range that can hear and understand you"
- Compulsion — "Creatures of your choice that you can see within range and that can hear you"
- Animal Friendship — "a beast that you can see within range. It must see and hear you"
- Vicious Mockery — "If the target can hear you"
However, Command has no such requirement. The only requirements are:
- the target must understand the caster's language
- the target must not be undead
- the command must not cause direct harm to the target
Does this mean the spell still works if the target hasn't heard the command word itself? Assuming the 60ft distance, this seems very plausible in many scenarios. For instance, the caster sets an ambush. When the caravan passes by, she whispers a command word "halt" to the caravan leader, in order to make the caravan stop right in the perfect spot for the attack.