Will the one who donated a microphone to the mosque be rewarded for it, even though it was not used and remained in his possession?

There is no blame upon you for taking the microphone from the mosque where it was endowed if that mosque does not need it, and transferring it to another mosque that is in need of it. This is because an endowment may be transferred for the sake of benefit, and you will be rewarded for this action of yours, even if the microphone has not been used yet. For indeed, a Muslim is rewarded merely for intending good.