There are a number of cheap devices which allow you to listen to radio signals, these devices were originally intended for watching TV, digital TV and radio signals through the computer. By using available software it is possible to tune into many radio broadcasts/signals, e.g. HAM radio, AM, FM, commercial airline signals, etc.
To take a look at some devices search for “sdr usb” or “rtl usb”, prices vary from around 5-20 pounds depending on branding / accessories.
If you are interested to listen to voices from around the world, some of these software defined radios are possible to use over the internet. This has the advantage that you can access better hardware and also listen to stations based across the world.
A list of stations is available here: http://websdr.org/