The process of recording a ghost's voice on tape is referred to as EVP, or Electronic Voice Phenomena. It's a concept that has really caught on in the past few years.
The idea behind EVP: Go into an area that could contain spiritual entities and literally have a conversation with the ghosts. Ask them a question, wait a few seconds, then ask another question. It's not important to actually hear the ghost's response to the questions. If you do hear a verbal response then your ghost hunt has just found gold. When you've finished asking all of your questions go back home and rewind the tape. Listen carefully from the beginning to the end for anything out of the ordinary. It has been reported in some haunting cases that the ghosts at the site have answered the ghost hunter's questions. You have to listen closely, the answers are not always clear, and may even be in a different language.
Many people have had success with this method however there are some serious problems that need to be addressed. One key characteristic important with EVP is the ghost hunter's own listening process. The tapes have to be reviewed and carefully listened to several times. The problem with that is that if you listen to anything long enough you can begin to hear things. You also have to be able to weed out all the background noises such as passing cars, barking dogs, the wind, etc. There's also the nature power of suggestion. You may want to give the tapes to a trustworthy, impartial listener. Ghost hunters have the tendency to hear what they want to hear from the tapes. They know what questions were asked and have an idea of what answers to expect. Because the technique's authenticity is impossible to prove no tape, regardless of what's recorded on it, can be considered hard evidence. Critics have a valid point when they say that the sounds or voices recorded are either natural noises or hoaxes. There's no way to prove that they're wrong. ]
Tips to Good EVP Recordings:
Use a full size tape recorder, not the handheld models. The small tape recorders are fine for witness interviews but not for EVP recordings.
Use an external, static free microphone.
Always use brand new, high quality tapes. Never record over old tapes. There's always a chance that the overlapping recordings will mix and you'll hear things that shouldn't be.
Don't bump the microphone and if you do, make a verbal note of it on the tape. You may also want to note any other nature sounds such as gusting winds, creaking doors, etc.
Record in 20 to 30 minute sessions. You can record longer, but keep in mind that you have to listen to the tape later. If you record two hours of tape then you have to listen to two hours of tape. That can be very time consuming.
Ask clear and precise questions.
Don't whisper during the recordings. You are not going to scare the ghosts away.
After each question wait at least 10 to 15 seconds before asking the next question. It won't do much good if you never let the ghost get a word in edgewise.
