Vazkii created a mod called "Ambience", which was a forked and updated version of his older mod "RuneTune" (that was made for 1.7.10). The purpose of the mod was to add in-game music via events the Player is performing, such as being chased by a Hoard, walking to a new biome, etc. What's unfortunate however is that the mod has been discontinued for a few years now, (probably due to the developer being busy in real life or focusing on improving other mods).
A couple of months ago, another user called "azazelthedemonlord", forked the project to create a new mod built on top of Ambience which would provide better transitions between songs. It would provide a fade away effect between changes songs. However, there was a bug where a song would fade in, play for a second or two, then stop. After that, it would fade back in and play normally. In this mod, I've forked that mod, and I have tried to patch the bug the best I have could. There is a new feature that also includes a randomized pause in between (in the code). It pauses in between transitions randomly with the minimum pause to be 3 minutes, 20 seconds, and the maximum pause to be 5 minutes. This mod is experimental and only used for the modpack, King Mammoth, to help fix the transition issue.
I am not an experienced Forge Modder. In fact, I am still pretty new to Forge, so there are bound to be some bugs in here. You shouldn't be expecting me to update a ton, because this is just here to help fix the issue.
You can find information about the original project page here: https://www.curseforge.com/minecraft/mc-mods/ambience-music-mod
You can also find information about Azzy's Ambience, a project I forked which included the fading: https://www.curseforge.com/minecraft/mc-mods/azzys-ambience
What means Verified?
Compatibility: The mod should be compatible with the latest version of Minecraft and be clearly labeled with its supported versions.
Functionality: The mod should work as advertised and not cause any game-breaking bugs or crashes.
Security: The mod should not contain any malicious code or attempts to steal personal information.
Performance: The mod should not cause a significant decrease in the game's performance, such as by causing lag or reducing frame rates.
Originality: The mod should be original and not a copy of someone else's work.
Up-to-date: The mod should be regularly updated to fix bugs, improve performance, and maintain compatibility with the latest version of Minecraft.
Support: The mod should have an active developer who provides support and troubleshooting assistance to users.
License: The mod should be released under a clear and open source license that allows others to use, modify, and redistribute the code.
Documentation: The mod should come with clear and detailed documentation on how to install and use it.
How to Install
Download Forge & Java
Download Forge from the offical Site or here. If you dont have Java installed then install it now from here. After Downloading Forge you can run the file with Java.
Lounch Minecraft and select your Forge istallation as Version this will create a Folder called Mods.
Type Win+R and type %appdata% and open the .minecraft Folder. There will you find your Folder called Mods. Place all Mods you want to play in this Folder
You are now Ready. Re-start your Game and start Playing.