[Drahken]

It used to be that cartoons had almost non-stop background music, but these days cartoons have very little music beyond the intro and outro. I can't help but wonder about when the change occurred (and was it very sudden or gradual), and exactly why it changed. The obvious answer to "why" would be cost, but was that the only reason? ...Or did people say "hey, we're tired of that, give us some quiet"? ...Or did creators feel it was better this way? ...Or was it all of those combined, or something else entirely?
I believe the changed occurred around the 2000s. 80s toons had constant music, and 90s toons had a lot, but I think 2k+ toons had very little.

[bowers]

 In the golden age of animation, the big studios all had house orchestras and composers who did background music for everything from feature films to Bugs Bunny. Must have been interesting for the musicians- playing swashbuckling backgrounds and cartoon chase scenes, possibly on the same day! Since they were already on the payroll, they could be used as often as the studio wanted and for any project, at no extra cost.
Today, all music is outsourced and audited for cost. My son worked for Disney doing exactly that job. Hence, less music in cartoons. Cheers, Bowers

[The Australian Panther]

While TV series still often have theme music, many movies and TV shows now use pre-existing music for the soundtracks and then put out a compilation soundtrack as an extra marketing objective. In some cases this is done quite well, as in 'Justified' and 'Hell on Wheels' to name two. And the 'Guardians of the Galaxy' movies.
You can find many modern soundtracks out there, most of them - although not all- instantly forgettable. 

[profh0011]

When I think of cartoon soundtrack music, 2 shows instantly come to mind:  JONNY QUEST (1964-65) and SPIDER-MAN (1967-70).

Hoyt Curtin did all the music for JQ, and had been the guy for most Hanna-Barbera shows back then.  His stuff really hit a high bar few have ever approached.

When SPIDER-MAN started from Grantray-Lawrence, they got a pair of big band composers involved.  Bob Harris did the theme song, while Ray Ellis did all the background music (some of which was based on Harris' theme).  This arrangement, funny enough, paralled that of the JAMES BOND movies (Monty Norman did the theme, John Barry did the scores) and the BATMAN tv series (Neil Hefti did the theme, Nelson Riddle did the scores).

However, when GL suddenly went BANKRUPT after only finishing the 1st episode of season 2, and Ralph Bakshi was hired to set up a new studio to do more episodes ON THE CHEAP, he resorted to using "Library Tracks" for the mountain of NEW music used in seasons 2-3, which was heard side-by-side with reused Ray Ellis music.

I found out about this in 2002, and was invited to join a Yahoo group dedicated to it in late 2004.  I became so enraptured by music from the KPM, Capitol & DeWolf "libraries" that I collected and put together dozens of CDs of all this "stock" music, and even assembled a few "custom" soundtracks of my own, for stories I'd written myself.

I've often thought since then, if I were ever in a position to direct a movie, I'd PREFER using pre-existing music, since that way I'd know EXACTLY what I was getting in advance.  I haven't really heard a memorable new instrumental movie score (or TV score) since the late 1980s!

[The Australian Panther]

I became so enraptured by music from the KPM, Capitol & DeWolf "libraries" that I collected and put together dozens of CDs of all this "stock" music, and even assembled a few "custom" soundtracks of my own, for stories I'd written myself.

There is somewhere a whole large site dedicated to 'Library' music. When my previous computer blew up, I lost the link. A lot of library music is now available on Disc and CD and is sought after by collectors.
This site has four volumes of Library Music
https://www.blowup.co.uk/records/releases/blow-up-presents-exclusive-blend-volume-1/
To quote,

Exclusive Blend Volume 1 is the first in a series of compilations reflecting the eclecticism of London’s Blow Up club, featuring obscure tracks that form an integral part of its musical make up. Volume 1 unearths recordings made for the KPM music library between 1968-1970. From 1965, KPM introduced writers from jazz and pop backgrounds into its writing stable, to contemporise its catalogue of theme tunes and incidental music. This album features the creme de la creme of these composers, including Keith Mansfield, Johnny Pearson and Alan Hawkshaw. For the trainspotters, an alternative take of Hawkshaw’s ‘Beat Me ‘Til I’m Blue’ (minus horns) to the version featured on The Mohowks’ L.P. ‘The Champ’ is of particular interest. For the sane among us, think of this album of just a little something to carry on the party in the afterhours.”
Paul Tunkin (Blow Up founder/Resident DJ.) 

Those of you who know theme music for 60's and 70's TV shows will know some of those names.
There are now four of these collections.
You will find most of Volume 1 here.
blow up presents exclusive blend volume 1
https://www.youtube.com/results?search_query=blow+up+presents+exclusive+blend+volume+1+ 
I hope, that for those not aware of 'Library' music, We have led you down an extremely rewarding rabbit hole.
Enjoy!