I finally got music to play from my iPhone through my PC's speakers. Sort of.
1) I followed the video by Ray Pastore on YouTube:
. This video showed me what settings to adjust.
2) Instead of using the audio cable shown in the video, however, I used an iPhone-compatible cable: the Belkin RockStar 3.5 mm audio cable with USB-C connector - SKU F7U079bt03. (I appreciate Spapakons for reminding me that " iPhones don't support all adapters." That encouraged me to do some research before buying anything.)
3) Initially, the sound was coming out of only one desk speaker. To have the sound come out of both speakers, I went to Settings \ Accessibility \ Audio and turned on the Mono audio setting. This Ironically, as the Windows description says, it combines "both left and right audio channels into one." The result was music coming out of both left and right speakers.
4) The problem is that you have to go through these steps each time you connect your iPhone to your computer. If you have an old iPhone, you can just leave it connected. But I use my phone every day . So, I decided that it's just not worth the hassle it to use this method each time I want to play music.
5) It might be possible to use the computer's Bluetooth connection to do the same thing. I tried downloading iTunes and Apple Music from the Microsoft Store. Although I was able see files and pictures I had on my iPhone, I still wasn't able to play music from it. At this point I'm not interested in playing downloading 3rd-party software.
6) The end result is that I'll either get new speakers that I can plug my iPhone directly into (I've checked and my current speakers don't have that option) or I'll buy a stand-alone Bluetooth speaker that I'll just use for my iPhone. Still, I've learned quite a bit about computer audio, so I consider this a good experience.