The extremely simple FM transmitter circuits you found have very poor performance:
1) The antenna is connected directly to the LC tuned circuit so the frequency will change if something moves toward the antenna or moves away.
2) There is no voltage regulator so the frequency will change as the battery voltage runs down.
3) There is no pre-emphasis (treble boost) like all FM radio stations have so sounds heard on a radio with de-emphasis (all radios have) will be muffled like your stereo with its treble tone control turned all the way down.
4) It is mono, not stereo.
I made the first circuit to see how bad it was (because somebody complained that it didn't work) then I fixed and improved it.
I added an output RF amplifier to isolate the antenna from the LC tuned circuit and give more range.
I added a low-dropout voltage regulator.
I added pre-emphasis so it sounds perfect.