the reason for this is because you must buy a 2.5 mm male to 3.5 mm female adapter to fit the headphones in the calculator link port. Because not everyone want to spend money on this nor search for one (these adapters are hard to find in some areas) just to be able to listen to music on calc, music/sound on calc draw little interest. You can use headphones from an old Xbox 1 too, by removing the plastic thing on the 2.5 mm plug, but then the sound only comes one way since there's only one speaker, and since Xbox is discontinued since 2006, these are hard to find today. Another solution is using an AM radio on a radio station with no signal, but the sound quality is terrible. Had the 83+ link port been 3.5 mm, sound on calc would be much more popular.

Another reason is that it slows down games considerably. If you search this forum well, there's a Guitar Hero demo with sound somewhere. Look how low the sound quality had to be decreased for sound to not take almost the entire calculator CPU usage. Notice how the few games with sound, such as Pyoro, only does beeps instead of long sounds/full music because otherwise the game would run incredibly slow. Notice how the softwares that actually can do music/sound while processing other stuff are for 15 MHz calcs only (like the Game Boy emulator for the 83+SE/84+ calcs).

Also, about RealSound: All TI-84 Plus calcs released after April 2007 cannot run RealSound because of a stupid hardware change by Texas Instruments in their calcs. This same hardware change, which is the replacment of the 128 KB RAM chip with a 48 KB chip combo, is what also causes Usb8x/Msd8x, Omnicalc RestoreMem(/Virtual Calc and TI-Boy SE to not work on any TI-84 Plus calculator released after this change, and even crash, in some case. Blame TI for this one. I don't see how they're gonna save money by using 80 KB less RAM in each calc.

I thought about doing a DDR game with sound before, or even a music maker, but since in BASIC the entire execution would stop during sound playback, it would be impractical.

I've tried to use Omnicalc's sound function in games, but the problem I hit was keypresses don't even register while a note is being played.

I think those are all good explanations.  The speed is probably the most major of those issues, but even when its not an issue, how many people have you seen carrying around headphones and plugging them into their calculators to play a game?  Its pretty inconvenient in my opinion since it doesn't really add much to games that are already so limited in color and resolution, it hardly enhances the experience.  I only added it to Pyoro because I thought it would be cool and I always wanted to try it.  Yet I don't even own a pair of those headphones.  I just spliced the link cable so I could bypass a speaker to it.

Axe will for sure have some type of ToneOut() command that will be like the pause command except play a frequency instead of doing nothing.  But that's about all you can do since anything more elaborate really takes a toll on the processing speed.  There is no hardware 'audio driver' so you must use software which makes it really slow.

I personally think the headphone themselves wouldn't be a big issue. Most people these days carry their iPod or mp3 player at school, so they would have headphones with them. THe real major issue about headphones is really the size of the I/O port. There's also the fact outside an ASM program, when headphones are plugged in, key response is horribly slow

If I remember, when not inserting the adapter completly in the link port, volume became twice louder, but if the sound was originally stereo, then it became mono. I wonder if this could damage the calc, though...

Well it's alerady optional to begin with, since if you don't have headphones you can still run the programs fine. You just won't get the full speed advantage of no sound support, unless the game has a no-sound mode, but then that makes the code larger or slower.

As for processor-heavy, in BASIC there isn't much games that would remain playable at all with sound. Even a RPG would run very slow, plus someone mentionned about keys not being registered during sound playback. This would make games hard to control. As for ASM, then I guess you have a good point, though, but if you notice, Pyoro, with just tiny beeps, is alerady running at a somewhat low framerate compared to many ASM games, and it's not the grayscale that slows it down because F-Zero has grayscale too and runs at like 30-40 fps, even if it has grayscale Mode 7 to render every frame. Also the TI-Boy SE sound argument won't work, because TI-Boy SE requires a 15 MHz calculator to run. It's way too much demanding for a regular 83+.

I think if you really want a game with sound that bad, you should maybe give it a try, because for many programmers, adding sound to our games isn't really rewarding in the end, since like 3 people will enjoy this feature, and it's a major hassle to add to games, especially in assembly.

One thing tho, in TI-Boy SE, when disabling sound, the speed gain is barely noticeable at all. But again, since the processor is 15 MHz, maybe it allowed Calc84 to multitask more, or maybe use interrupts or something, to allow sound to run without slowing the rest down. Sometimes, 2.5x higher MHz can make a big difference. If I remember, there was a SE-only version of the GH clone and it had much better sound quality than the other one. Given how long sound has been done for calcs so far, though, there is definitively a reason why it was not used that much on z80 calcs yet, even if we had 3D and other advanced features for ages.