I have had the same issue with my inspiron 11 3000 series. Very annoying. It has the Realtek HD sound system.
Between all the bugs in this computer's OS and Widows 10 this has been the worst computer I've ever owned. What happened to the old Dell? They used to make a reliable product with great customer service.
It sounds like that they may a program assuming control of the sound. Right click on the speaker icon and choose playback devices, and choose the sound device that you are currently using (likely Realtek speakers) and click the properties button. Click the advanced tab and then look in the middle of the screen for exclusive mode. Uncheck the two checkboxes listed (allow devices to take exclusive control of this device and give exclusive mode applications priority). Now check to see if the volume jumps up and down.
Specialize in Laptops, Mobile Devices