I am posting a message from David from Australia here, who did some successful experiments with on-board sound:
This turned out to be quite simple. An SD card is not required and the sounds are stored in memory.
The procedure is:
1. Partition off 1 MByte of memory for storage. Tools/Flash Size/MB(FS:1MB OTA ~1019KB)
2. Install LittleFS library (SPIFFS is depreciated) Library manager or https://microcontrollerslab.com/littlefs-introduction-install-esp8266-nodemcu-filesystem-uploader-arduino/ ( Note – This only works with IDE 1.8.19)
3. Upload suitable sounds to Memory https://github.com/dagnall53/WiRocS/commit/df609b8b5e5c7882288a7a050cee5dd87a627c20
4. Install a suitable ICS module. I used a MAX98357A from Adafruit.
5. Install ESP8266Audio. Sketch/Library manager
Sound quality is quite good at low gain but distorts a bit at higher volume. Possibly due to the $2 speaker I was using!
I found some very interesting software at https://github.com/dagnall53/ESPMQTTRocnetSound shows what can be done, apart from simple whistle/horn noises.
Thank you, David, for this contribution.