I just started having this problem on my iPhone XR. I thought I had fixed it yesterday but the problem is back today. Interestingly, I have made a discovery and I think the problem is somehow related to video playback. I was just engaging in a text conversation with a friend this morning and noticed my text alerts weren’t working, again. While making my usual social media rounds I went to Instagram and clicked on a video. There was no sound so I pushed the button to increase the volume. The video played with sound and, lo and behold, I received a text with its alert sound. I remember this happening yesterday too, which is why I thought I fixed it. I had been engaging in a lengthy text conversation then, and wasn’t getting alerts, so I had to sit there and watch my phone for the texts to come in. I went to YouTube to play a video and the same thing happened; no sound. I pushed the button on the side to raise the volume and instantly the texts were coming through with their normal alert sounds. I had already run through the gamut of troubleshooting solutions to no avail. I did a few reboots, made sure the volume was set properly and made sure all sounds were set to the on position. Banners as well. Everything was perfect but every time I go to watch a video I have to adjust the volume manually to hear it because there’s a little phone gremlin who keeps turning the volume down. For me, the temporary solution is to watch a video and manually raise the volume, as it’s always set to zero despite what the settings indicate. Bizarre.