One of the things that comes up often with new students is their inability to see and hear spirits. The first thing I ask is if they've been doing their meditation and contemplation exercises. I cover it briefly in the first lesson of the Black Work course, and I wonder sometimes if I've under-emphasized how important the role of meditation and contemplation is when it comes to scrying.
I've said it a number of times over the years on this blog, but it bears repeating. Spirit voices don't sound like an audible voice. Once in a while they do, once in a very great while, but most of the time the voices of spirits sound a lot like your own thoughts. They just come from outside your head. It's easy to ignore the voices of the spirits if you don't know "where" in your own brain your thoughts and images originate. The only way I know of to learn to differentiate between your own thoughts and the thoughts of the spirits is through meditation/contemplation.
The trick isn't shutting down your mind and sitting in completely perfect zazen, attaining no-mindedness, although it's valuable to strive for this state as well. It's not about learning to hold a visualized image in your mind for long periods of time either, though that too is useful.
The trick is observing your thoughts. Where do they come from when they arise? Where do they go to when they fade and your awareness moves to something else? I think the book that helped me understand the art of meditation and contemplation the most was Heartdrops of Dharmakaya. I really can't recommend it enough.
Once you're familiar with your own mind, your own thoughts, and "where" in your head your thoughts originate, you'll be in a better position to recognize the voices of the spirits.
Even with meditation and contemplation exercises, it takes a while to get used to the difference between your own thoughts and the spirits' voices. At first it can be scary, because people have a healthy fear of deluding themselves. That's awesome. I find most of my students with issues scrying are just afraid to trust what they hear as being valid.
The best way to get over that experiential hump is a magical diary. You write down what was said, what was heard, and you check back later to see what the results were, and whether the conversation turned out to be useful. Your own mental chatter during a rite will be apparent when nothing the alleged spirit has said turns out to be useful. At the same time, you quickly learn to "sniff out" the voices of the spirits, and the things they teach and say are meaningful to you in your pursuit of the Work.
The diary gives you a place to exercise your healthy skepticism, and a means of growing in confidence. You have a record to check to see if you're crazy or actually getting anywhere. It's a useful tool.
Plus, they're a lot of fun to go back and read years later.