what i'd recommend is to take some inspiration from Empire Earth II, which also had a capturing feature (though 0ad's functions is much better). in EE2, when units (just infantry in that one) are told to capture a building, they walk up to it and essentially start saber-rattling, raising their weapons over their heads and shaking them as it looks like they're shouting (but there's no special sound for that). for 0ad, it could be done with cavalry by their horses rearing up, and units with shields could bang their weapons against them.