You've probably heard it. A vocal in a song suddenly sounds like it's coming through a telephone or megaphone.
Several songs that use this effect in at least part of the song come to mind. Here are a few: Telephone Line (just at the beginning) by ELO, Video Killed The Radio Star by The Buggles, Convoy by C.W. McCall, Going Under by Evanescence (just at the beginning).
So how do you make a vocal sound like that? You can get that effect really easily in any audio . The link below from Reaper Blog though. So that video will walk you through how to get the megaphone vocal effect using Reaper.
They mention a program called McDSP Futzbox, which is a paid (~$129) plugin that creates lo-fi effects. But you can easily do the same basic thing just using Reaper's free plugin calledReaEQ, included with REAPER.
Basically what you are doing to make any audio sound like it is coming from a megaphone (or telephone) is emphasizing the middle frequencies and pretty much deleting the low and high frequencies. You can just create a narrow bandwidth in the middle of the frequency spectrum, and filter out the rest of the frequencies with the EQ plugin. See picture at the top.