NO! NO THEY ARE NOT!
Magnetism is the introduction of a field effect, whereas Gravity is the irregular warping of space-time, which happens to most closely match a field effect as opposed to a point effect.
So... Plasma then? (BTW, this change makes a lot of sense. It's just that we already have that element...)
Ice is the second element I would remove (after stone) because of this - there is no such thing as 'cold.' If a toa can manipulate cold, then they are manipulating the heat, and just removing it. For example, Tahu can freeze areas by removing heat, and inversely Kopaka should be able to warm areas up by 'removing' cold (adding heat.
I think sound is a really cool concept, but yes, this is true. I mean, a toa of sound may as well be a very skilled toa of fire, (or heat), as sound is simply moving particles in waves - therfore a more skilled toa of fire. Perhaps it could be introduced as a different style of fire? As in,Tahu learnt to manipulate heat in a powerful and physical manner, whereas the toa of Sonics learnt to manipulate heat in a manner whereby they exert a fine control, as opposed to being uber-powerful.