Actually, the interior of "quiet PC" cases can come with foam sheets that you cut an apply to the interior panels as part of the sound-reduction installation process. There are also foam blocks that you cut and insert into empty 5.25 " and 3.5" drive bays, just to reduce interior noise exiting through openings on the front bezel.
Another well-designed case feature is to have a washable/removable screen mesh (to collect dust and other air-born animal hairs, etc.) on the front bezel interior, over the front-lower case chassis intake fan (which sucks cool outside ear into the case interior lower portion so that it then rises over the interior cards/drives before getting blown out the upper-rear by the exterior exhaust fan mounted on the rear of the chassis. Every so often you remove the front bezel, remove the "dust shield screen" from the interior of the bezel, wash it thoroughly, reinsert it inside the bezel, reattach the front bezel to the case, and you've got your original unimpeded airflow back.
If you put your case on your desk right near you, noise is (for me anyway) a VERY important criteria in choice of the case and also fans. The latest computer I just built about 8 months ago (based on an ASUS P8Z77-V Pro board) uses the same
all-black AcoustiCase case I'd purchased some years back specifically for its "quiet PC" characteristics, which is remarkably silent (in terms of letting interior noise escape). This super-silent machine has four internal hard drives (three 7.2kRPM SATA drives and one 10kRPM SATA drive) along with
120mm Noctua NF-P12 120mm fans at front and rear that are essentially silent.
It also uses a
600W Nesteq "modular" power supply, which is very convenient since you only need to plug in cables to feed equipment you actually have, thereby reducing interior "cable clutter" from power supplies that simply come with a big bunch of pre-attached cables all tied up in a bundle. The Nesteq power supply is again very very quiet.
Then, the
Noctua NH-U12P CPU cooler I bought for the Intel i5 3350p (no onboard graphics) CPU was also from Noctua, basically a large heatsink sandwiched by a pair of identical NF-P12 PWM fans (arranged in push/pull orientation). Essentially silent again, and remarkably effective in keeping the CPU cool.
So it's really what your requirements are, that determines what equipment you seek out and purchase. In my case I'm not a gamer but I wanted a "silent PC" because I sit at my desk and work for very long extended periods.