You can set the audio volume with v and retrieve it with V - its in % so ranges from 0 to 100
There are two audio paths on the FONA. One is the headset, thru the 3.5mm audio jack. The other is "external" - using the two speaker and mic pins for wiring up external speaker and mic. FM audio, phone calls, tones, etc can be routed to one or the other.
To set the audio to headset, use the command H
To set the audio to external, use the command e
You can test the audio path with the toolkit tones. These are tones that mimic what some phone services sound like. For a full list of tones, you can check the AT+STTONE command in the AT command datasheet. We'll use tone #20 which is the American dial tone.
You can switch to headset mode, play a tone, then try it on the external audio mode. This is a very easy way to try out both speakers for debugging