Sound : optional SN76489AN

[snake]

Why was this chip optional, was it an expensive chip at the time?

Can it be fitted to ALL Microbee models?

What games used it?

[someone]

The Texas Instruments SN76489AN PSG and accompanying audio amplifier were added to the Alpha Plus prototype (aka Premium Series) as a feature bump.
However when it was sold both of these features remained unpopulated. Prior to that the General Instrument AY-3-8910 was sold as the BeeThoven expansion accessory.  PSG were not overly expensive but the microbees were made to a price point for the NSW Education department thus omitted.

Other features that were on the prototype and not included in the sold model include:
  Texas Instruments RS232 transceiver
  Motorola MC146818 RTC (Real Time Clock & RAM) 
  8K Char, Attribute & Colour RAM

As for software for the SN76489AN, there's a Jukebox program that plays 3 channel music.
Someone doesn't know whether the MWBASIC was updated to support it.