To elaborate a bit.
A locator / outer marker has two different transmitters: An outer marker beacon, and a low powered NDB.
The marker signal is designed to have a very narrow width along the localiser course but fairly wide compared to the narrow orientation. This is to give a quite accurate position fix as the aircraft moves along the localiser, and still have the same effect if offset from the localiser a bit.
The nature of the beacon radiation pattern, and that of the localiser, means that it is not feasible to navigate to the localiser course to commence the approach from most azimuths, so...
...install an azimuth aid eg VOR, NDB that would allow the aircraft to be navigated to some point on the localiser that is suitable to commence the approach. The simplest - and cheapest - is to use an NDB. Low powered because it is intended for terminal navigation, not en-route - hence the term 'locator' (from 'compass locator' which was used to distinguish it from a 'radio compass', the old term for an ADF receiver)
There are different types of marker beacons: Outer, middle & inner. They serve to give position fixes along the localiser, something that is now usually done via DME or GPS. Marker beacons operate on a single, fixed frequency, and each type of marker beacon has a particular tone, ident & light colour associated with it. This lets you determine which marker you're crossing over.
Locators, being an NDB, albiet a relatively low power one, operate on various frequencies selected when installed so that each one doesn't interfere with others. This means that each locator, just like an NDB, needs a unique identification.
The identification used is an aural morse code. Trouble is, that aural signal has to be generated somehow to reflect some change in the base NDB signal that matches the desired morse code pattern.
One way to is have a plain, unvarying signal, and then use a some circuitry in the receiver to add an audio tone when it is receiving that signal. This is called a Beat Frequency Oscillator (BFO). If the locator (or NDB) signal is interrupted in such a way as to produce a Morse code pattern in it on-off-on-off pattern, then, in concert with the BFO you would hear a tone that matched the locators interrupted signal.
An alternative way is to have the locator's signal modulated just like an AM broadcast station, to allow an AM receiver to detect the amplitude difference and match that difference to an audible tone just like an AM broadcast. Not a voice audio though, just a beep when the amplitude is above a threshold, and silence below the threshold.
If the signal is further adjust in amplitude in away that is proportional to sound, then it would be like an AM broadcast radio station, and you could have additional information transmitted. Hence the with voice or without voice annotation.