Here is a video by Streaky (one of the best mastering engineers in the world) who talks about money and music in 2017:

ROYALTY FREE MUSIC – that is when you you are paid once and the client can use your piece of music in his/her product and owes you no money(royalties) for the sells of the product. These days film and TV companies still spend big money for music but they work with limited number of names. However in the internet age there are thousands small  website developers, video bloggers, youtube stars, small ads producers that all need music in their work and they do not have the time, money and knowedge to calculate royalties so they buy music and sound FX from online royalty free music online music stores. There are few of these and if you want to make some $ from your work you can use them all. One of the biggest at the moment is

The down site here is that you really have to sacrifice much of your creativity and to think of these tracks as something that will be used as a background. That kind of music is not the center of attention and is not the main feature of the products where it is used. However you can build a name that way while making money and you can still make your creative tracks. Just keep them separate and use the royalty free music markets to rise the attention to them.