Low sound on phone

I know it sounds mundane, but have you cleaned out your speaker ports? It is something that easily gets overlooked, occurs frequently due to how often phones go in&out of pockets + depending on phone case design may occur more often, plus it doesn't help that depending on how the "clog" forms and/or how long it's been in the port --> even a more-than-reasonable "once over" in slightly suboptimal lighting can result in the port(s) appearing to be completely fine.

If the ports have been cleaned --> Unfortunately we're likely talking a hardware issue then (unless you're using a jailbroken / previously jailbroken device since code could be sabotaged somewhere then). Nonetheless, your best bet for a fix may be just to find a replacement micro speaker online for a couple-few bucks for your phone model, open the phone up, remove, and replace the speaker. Bonus Trick: If you do not have soldering skills and/or proper tool access, you could take it to a small shop phone repair place; it'll just cost a bit more!