Using Music
Behind my DMs Screen there are many things, one of the most fun is my tablet because I can give my sessions a backing track.
When picking tracks to play I look for something I can leave running for a while without having to loop it and for the music to match the group I'm DMing thematically.
For my Forgotten Son's campaign, which started in The Sunless Citadel, I got the sense that they were more of a nature/classic fantasy group so I'm currently using Celtic music as their backing track. The next step was to find a gentle regular backing track for general use, a tense track and a stirring/upbeat track for combat.
Once you've got your music opened in different tabs so you can flip between them you can start to unconsciously affect the players' mood.
Remember to change up your songs as they can get tired. For my Monks of Tyr (Asian Themed) campaign I use NHK Blends as fight music because the group is quite lighthearted and I enjoy their reactions when they realize what song they have been listening to!
You can also experiment with letting you players pick the music if they are top of the initiative order or take it in turns to do the combat music on a session by session basis.
When running the Sunless Citadel I used different music for different levels of the dungeon. The top level was the Celtic but I used ambient horror sound effects on the Grove level to change the tone to a more creepy setting. This playlist is great:
There are loads of youtube channels devoted to instrumental music, share your favourites below.