The Harmony should be able to handle a lot, there is at least also an iOS and Android App.

Have you ever tried a mobile app to "only" control the volume on your Mac?
I think Harmony is overkill for that (shooting at sparrows with cannons).
There are many (free) apps to control media centre / volume / channels / movies etc. via smartphone.

Of course depending on the infrastructure (e.g. via WLAN / LAN / Bluetooth etc.), player etc.