This is a great intercom circuit that can be used in many ways. It uses 22V to operate and maybe it will work at a lower voltage (you can try it). For input/output it uses a loundspeaker (20-45 Ohm) on each side.
Click to enlarge
|R1: 2,2 M||C6: 200uF/40V|
|R2: 10 Ohm||IC1: PA234|
|R3,R6: 100K||LS1,2: 20-45 Ohm|
|R4: 22K||T1: 2N3391 (BC383)|
|R5: 1M||P1: 5K|
|C1: 0,2uF||S1: SW-DPDT *|
See here how the SW should be. See also the large theoritical image.