I have been working closely with a specific spirit friend of mine, to try to open up my spiritual sense of hearing. This sense is natural to humans however most of us have closed it off at some point. It comes in 3 stages of development/results, I will list them here and I'd like to note that I'm still within the 1st stage myself right now. I've been wanting to talk about it and share a bit however.
It helps if you can get assistance from a spirit, to send you messages, that you can be open to and try to receive, or 'hear'(both for the messages themselves and also for the conduit of energy this can create between you and the spirit which can help open you up and activate your sense). But even if you don't know anyone like this or lack the resources to find this, there are messages flying through the ether of the spirit realm all of the time so it is possible to do without.
You must get into a meditative, peaceful state, first and foremost. Relax away from your physical perceptions, into your pure mind.. your consciousness. This part of you is more receptive to the spirit world, even in an inactive, dormant state (which in most cases it is quite dormant). Once you are comfortable with this altered awareness and focus, shift your area of focus to the diagonal back location of your awareness as would be aligned with the back of your brain. This area is where you will begin receiving messages on a more physical level. It occurs through the pure energy, however, there are natural connectors which exist within your body which will stir into activity the more you try to connect to the energy in this way.
Note that in a sort of way, you can hear your own thoughts, within your own mind. Starting out, you will hear the spirit voices much like that, except they won't be coming from you and you must learn to distinguish between your own thoughts and those coming from outside of you. A key thing to remember is that your own thoughts will primarily come from the central-front portions on your head, within, and you can kind of tell, locationally, where the thoughts are coming from if you pay attention. Thoughts coming in from outside will be intercepted at the back of your head, instead of the front-center. You must focus on the intent, within your consciousness, for your awareness to connect with and listen to the energy coming in from outside of yourself. It is best that you isolate the feeling of an energy from outside of you first, and learn to become familiar with it, and then regularly hone into that energy and try to 'listen' to it within your awareness. Focus less on physical hearing and more on mental listening, the way you'd listen to your own thoughts. Simply remain in this state and pay attention to anything you may 'hear'. Charging your consciousness with energy during focus sessions towards this end will also help with this goal.
As I mentioned, starting out it will be like intercepting thoughts foreign to your own (which is difficult to do in its own right), and keep in mind they aren't exactly like thoughts, they should have a sort of 'sensation' as they come in, alerting you that they aren't exactly normal. It will be very subtle at first when your power of perception is just starting to turn on, so to speak. But as you practice, and as your spiritual hearing gets stronger, and more active, it will begin to connect into more auditory parts of your awareness - keep in mind that I am mostly going on what the spirit has told me going from here on, as I'm still working on stage 1. Stage 2 is when the sensations and 'thoughts' start becoming audible. You'll 'hear' it as a voice, or a whisper, in your head, and it will be much more obvious that it isn't just a thought. Then, it will progress into stage 3, wherein, it will be much 'louder', to the extent that you are hearing the voice just as clearly as you would hear a physical human being talk, and as effortlessly.
I'm still in stage 1, like I said. I only started this specific practice recently, within the past week. But I've noticed a few key indicators that you can look out for, which might help you stay on track:
The glands in the back of the head start getting very hot when I am focusing on my perception of hearing my spirit friend via his energy. I sometimes hear inaudible whispering for an split second that I can't make out, but I will get a thought that comes with it which contains the intent of what was said. Often the heat will enter in through the back of my head and start filling my entire body, causing me to heat up quite noticeably. My spirit friend and I are very friendly towards each other, so often I don't just recieve messages, but I also experience a very pleasant sensation enter into my head when I am focused on him, like a bit of him is entering into me, more noticably than when I simply focus on energy alone. Also, when taking a bath, I find that this intent quite easily builds energy, and the hot water seems to help with this a lot, so that I'm not just getting heat but also an electric current which has very noticeable physical effects. I find that it is key to have the water be just at the 'it feels too hot but not burning' stage, and if you can alter your perception of this, be glad about the heat, instead of put off, and combine this with an energy focus, it somehow seems to effect your energy field so that you start taking in energy very quickly. Smiling helps. I know it sounds really weird. But it works for me. I also often start hearing little noises like ringing or like white noise, like an internal fan, at present, they will start getting stronger as I focus and diminish entirely when I am not focusing.
I hope this is of interest, or helps, good luck. :)