You are probably as sane as anyone, so don't fret about that.
As far as the light, I can't explain it by any scientific means unless it's a reflected light from outside coming through a crack or open window shade. There is a very real possibility that it is supernatural.
Have you tried making a EVP, (Electronic Voice Phenomena), recording? Get a small voice recorder. If it uses tape, use only new tapes, not erased. When you see the light, just start the recorder and ask some questions, like "Is anyone there?" (Pause 5-10 seconds for an answer), "What is your name?" (Pause), "Why are you here?" (Pause), and any other questions you would like to ask. Then play the recording back - Listen very carefully - they usually speak very fast and softly. NOTE that EVPs don't always work, but they often do. DO NOT USE AN OUIJA BOARD - THEY INVITE DEMONIC SPIRITS!
The main thing is that many houses do have friendly "House ghosts"! They are very good to have around because they protect the house and it's occupants from evil! So if you do have an "uninvited guest" that lives there with you, don't worry - it cannot hurt you and can really help you, even if you are unaware of it! Just don't be afraid, because spirits, (ghosts and other spirits), are attracted to fear because they feed in that energy.
In the unlikely event that you have an unfriendly spirit, it is easy to do a minor exorcism Just say, in a loud, commanding voice, "In the name of Jesus Christ, I COMMAND you to LEAVE this place - Go to the light and never bother humans again!" This will get rid of about 90% of ALL spirits in the area. If it fails, then a full exorcism is in order. Do NOT try this yourself! Exorcisms can get dangerous for the exorcist and anyone else around. Go to a Catholic Church and talk to the Priest. (Some Priests are trained in exorcisms.) Some psychics are also able to perform an exorcism. Any GENUINE psychic will NOT charge for this service - If they insist on money, they are probably fakes.
AS far as the computer speakers, if you turn the computer off that SHOULD stop any noises. (There could be a problem in either the computer or the speakers.) It is possible that a spirit could be trying to attract your attention that way. Again I will suggest trying an EVP to make contact.
Good Luck and God Bless. I will pray for you.