很高兴为你解答.
A microphone can make your voice (sound) louder.
He felt happy because they all chose him (to )be their monitor.
或者：He felt happy because they all chose him as monitor.
解析：make sb do sth 使...做,make sth done使...被 ,make sb/sth +（感官动词原型,可省略）+ adj 使.怎么样.choose sb as 把...选为.（职务用零冠词）,choose sb to do 选择...做.
祝开心~!