An AY chip connected to an EP can be a must, but.... Why not fit an AY compatible chip with more options?
Based on the AY's General Instrument series of sound chips, Yamaha released its own series of chips oriented to the Arcade and PCs markets.
First they launched the YM2149, a forty pin exactly clone with the option of working with selectable single/double frequency, putting pin 26 free or to ground.
Secondly they released the YM2203, YM2610 and YM2608, adding 3, 4 and 6 FM channels to the AY architecture. But, in order to play analogue that digital sound, the adition of a companion external DAC chip(YM3014) was needed.
The YM2203 is exactly an AY 40 pin clone(YM2149) with the added mono FM synthesis. YM2610 and YM2608 are still signal compatible(TTL), but have 64 pin packages because they add ADPCM mono synthesis and stereo output on the FM side.
Sadly, further development of the Yamaha sound chip series lacks the AY compatibility.