Are the opinions, ideas, and voices of junior enlisted ever taken seriously by senior leadership?

Ignore the opinions and suggestions of inferior leaders at your own peril. I once warned my superior's that they were taking an unnecessary risk. They agreed that it was a risk but felt that it was acceptable. When I disagreed, they told me to write a Memorandum for Record which I did. When the risk was realized and their superiors came looking for blood, I clad myself in that MFR and was immune to the buck passing. To the general's credit, he took his punishment like a man and then rewarded me for my attempt to save him the disgrace.

True leaders always listen.

Absolutely! I've been in some exceptional units and some, not so much. In the exceptional units the senior leadership made it a point to get out a visit the troops on a regular basis. To personally look them in the eyes and ask for and listen to the feedback of the Jr. enlisted. In-line with what SN Greg Wright mentioned with regards to the ASVAB scores, in today's all volunteer military, there are many Jr. Enlisted with under grad and graduate degrees. That said, you don't have to hold a degree of any kind to have a great idea or suggestion. The enlisted corps is the back backbone of our military and in many cases they are the experts in their field. A good leader would be foolish not to take them seriously. Personally, I will always listen to suggestions/recommendations from my Airmen and NCO's, some I will take and run with, others I might not, but I'll always take into account what they have to say.