The sound comes from a heavy circular block of material with a hole in the middle covered by a reed.

In really old mamas and growlers the block was attached to a thin paper diaphragm which is compressed as the block falls under gravity, air from the compressing diaphragm is bled slowly through the reed which makes the noise.  Over time (or if a teddy is washed) this paper deteriorates and splits and the block just falls quickly with a clonk as the air escapes quickly from the ruptured diaphragm.  It is usually very difficult to find paper thin, flexible but strong enough to fashion a new diaphragm.  We do know of a US company who sells diaphragm material (Mulberry paper) - see bottom of the FAQ page at http://id.mind.net/~truwe/tina/parts.html

In newer models, the block has a thin circle of soft rubber around the outside of it which forms a seal onto the circular case - so as the block falls under gravity it falls slowly as the air bleeds through the read and you get a long note ("maaaaaa" in the case of a mama).  What usually happens is that all that scraping down the inside of the mama or growler case wears the rubber away - and again the block just falls quickly with a clonk as the air below it rushes around the block. We've not found a way of replacing this very thin and flexible rubber seal (although Shona Gladwin has - see below).

(In a mama, as the block falls, a peg (plastic or wood) attached to the mama top gets momentarily caught as it grabs onto a circular hole of paper or rubber stuck to the top of the block. This short slow down gives the "Maaa Maaa" noise.)

So, given the low cost of the replacements it is usually far easier to just replace the unit.

The problem comes when the replacements are slightly too large (usually mamas in dolls - teddies tummies can stretch a bit) and then we have to resort to various means (rubbing down the mama cover that holds it, opening up the hole where it fits etc) to MAKE the replacement fit.

Shona Gladwin has just sent us a great tip.   Shona reports:-
"I have been able to fix old mama voice boxes with a circle cut from latex hospital type gloves fixed to the cylinder with cycle inner tube repair glue, I also used condoms cut into circles fixed with the same glue".  

Great idea, thanks Shona - we must try that next time we get a dead mama that needs a new seal around the falling block.