Lurking the official forums yesterday, I came across a issue where players were experiencing problems with the hot tub and the fun level bar decreasing instead of increasing.
This is not a glitch or bug. If you look in the build/buy mode, under plumbing, click on the hot tubs it will show that "fun" is not in the description like environment or hygiene.
How do you fix it? You're going to have to upgrade it to have the stereo. Which means you will also have to have a sim with a high enough handiness skill that allows them to upgrade objects.
*To upgrade the hot tub to have the stereo, your sim will need: 3 common upgrade parts
*To upgrade the hot tub to have the aromatherapy, your sim will need: 3 common upgrade parts and 1 plumbing upgrade part
***To see if your sim has the required parts, check in your personal inventory after your sim has repaired or salvaged an object/repair refuse pile to get parts.****UPDATE****
The Sims team is aware of the fun level not rising and is currently working on a patch to fix this issue.