In an era of ubiquitous digital devices with built-in microphones and recording capability, distributed microphone arrays of a few digital recording devices are the emerging recording tool in hands-free speech communications and immersive meetings. Such so-called ad hoc microphone arrays can facilitate high-quality spontaneous recording experiences for a wide range of applications and scenarios, though critical challenges have limited their applications. These challenges include unknown and changeable positions of the recording devices and sound sources, resulting in varying time delays of arrival between microphones in the ad hoc array as well as varying recorded sound power levels. This paper reviews state-of-the-art techniques to overcome these issues and provides insight into possible ways to make existing methods more effective and flexible. The focus of this paper is on scenarios in which the microphones are arbitrarily located in an acoustic scene and do not communicate directly or through a fusion centre.