If your laptop is "hacked" then everything you do on the laptop can be known to the attacker. Yes, the microphone can be used to listen to the environment sounds and there is no ambiguity about it; the debate on Webcams is because Webcams often come with a LED which supposedly turns on whenever the Webcam is "looking", and the question is whether it is possible to activate the Webcam without turning that LED on. Microphones never had such a LED to begin with, so there is no question about that: whatever controls the computer can "listen" at will.
The same can be said of your cell phone or of any system which runs software and physically contains a microphone (so this also applies to most "land-line phones").
Low-tech solutions have the nice side of being obviously right, meaning that you can, as a human user, check that they are in force, without having to trust that the machine was not compromised as some software level. The trouble with microphones, though, is that while deactivating them permanently is easy (if only by wrenching the microphone out with a pair of pliers), a reversible block is hard. One possibility is to physically disable the internal microphone, and plug an external microphone in the relevant plug whenever you actually want to record sound.
Apart from that, a machine compromise is already quite pervasive: if hostile entities can activate your laptop microphone without your consent, then it can be argued that you already have bigger problems, namely that all your emails, documents, network activity, passwords... are known to the attacker.