There are two ways that spirit voice can be channelled, one is commonly used by mediums who have developed clairaudience which is just simply passing on what the spirit person is saying to them. The other way is the spirit person actually using the mediums vocal chords to communicate with, this is not something that is easy to do although can be developed with a lot of patience and time.
I will give you a simple exercise to help you develop this, but this is something that can take years to develop so don't expect too much to happen. This exercise shouldn't be attempted alone, Ideally this would be best done in development group where you can have an experienced medium at hand in case any problems should arise.
You'll need to be sat on a chair that has a back support, sit with your feet flat on the floor and hands resting face up on your legs and close your eyes. Next take a few minutes to get comfortable and relax, a few deep breaths can help. Ask your guide for protection, if you don't know your guide at this stage just mentally ask for protection, whoever is with you will hear your thoughts, (there is a protection exercise on the "Psychic Protection" page) also tell your guide your intentions for example tell your guide or who ever is with you that you intend to channel spirit voice.
When you're ready start to imagine that you have a funnel which is connected to the top of your head, visualize this funnel as clear as you can and make it a silver colour. Hold this vision for a few minutes, then imagine someone pouring white light into the funnel until it fills up, when it's full hold that vision for a few minutes. Next let this white light flow right through the funnel, right through your head and out through your mouth, until all the white light has passed through. This creates a subconscious channel and because the light is white it will keep the channel clean and pure. After you have done this sit quiet for a while and see what happens, like I said above this is something that can take years to develop so don't expect too much first time.
When you feel you are ready you can repeat the process in reverse order by imagining the white light coming in through your mouth, through your head and back out through the funnel, then imagine your self removing the funnel from the top of your head, then you can close down by imaging that you have roots coming out of your feet which are firmly rooted into the ground. When this is done open your eyes, you may just want to sit for a while as you may feel a little light headed before carrying on with anything else.
Always use protection before attempting any psychic/spiritual exercises.