I'm not an Amazon Prime member but I do have an Amazon Echo. This morning I asked it to play me a track from my music library and was immediately given the message that I was on a 14 day trial with Amazon Music Unlimited. I immediately checked my subscription settings and found this message:

"As an Echo customer, you get 14 free days of Amazon Music Unlimited, which expires on 17 September 2018. You will not be charged at any point. For a limited time only, start an extra 30-day FREE trial by saying, "Alexa, I want Amazon Music." After the trial, your subscription renews monthly."

So it looks like the 14 day trial is completely free and I will not be charged at the end of it, nor automatically subscribed to the service. Only the 30 day free trial will trigger that.

So, if you have an Echo, take advantage. You'll be able to request anything that Amazon Music Unlimited can supply.