iTunes volumes decreases permanently after receiving notifications from Mail or Messages

Using a MacBook Air (2008) running El Capitan. I have iTunes running, playing a video, and when I get a notification from Mail or Messages (even if I'm sending the message, the sound that's made when it's sent), the volume permanently decreases, and I need to manually increase the volume. Restarted iTunes and then Mac. Any one else having this issue or have a solution?