I have seen the inputs change on their own too. If you have a monitor that you hook up with hdmi or display port, and your video card supports it, HD audio could switch to the monitor speakers, for example, on a driver update. It would help to know your set up, MB sound? Sound card? etc. Also what the hook up was, audio jack (minty green), optical, etc. What speakers... it sounds simple to ask, but there are so many variations it is impossible to troubleshoot without details.
For example, I have an Asus P8Z77-Pro V MB and I am using the on board audio. I have it hooked up to an LG soundbar using an optical cable. I have a Gigabyte GTX 1070 video card with HD audio, but I don't install the drivers because I had them take over and mess up my sound before. I use a Display Port (DP) cable to hook up the sound card to the monitor, and my monitor's on board speakers are turned down to zero.