I have been trying to use Alexa on my Raspberry Pi 3 B+ (running Stretch). I ran Alexa successfully, with a USB microphone and a speaker connected through the 3.5mm jack, about a week ago and went on to make a python program that does not in any way tamper with the 3.5mm jack or the audio driver. Today I started Alexa again and there was no audio output. I connected to the Pi through VNC and found that the sound icon on the top right of the screen is muted. I clicked the icon and the drop down menu says "No volume control on this device." I would like to know the cause and solution of this issue. I have searched for solutions through Bing but had no luck.
Also check the /boot/config.txt file. It should look like this:
# Enable audio (loads snd_bcm2835) dtparam=audio=on
I suggest you run raspi-config, look at Advanced Options>Audio and make sure the "1 Force 3.5mm ('headphone') jack" option is selected.