This is a Hardware Fault that will get worse, it's caused by a manufacturing defect that was present when you purchased the Phone.
This is not a software issue or network issue.
You should request a full refund or Brandnew replacement regardless if the phone has warranty or not.
Apple are blantantly avoiding this issue so they do not have to recall.
There is a Clear Issue here, why don't you research the Problem.
Thousands of people are reporting DIY repairs for Loose Solder Connections, Search the forums there are countless Instances of this issue, bear in mind these are only an minute proportion who can be bothered to sign up to apple and report it on the forum.
When i have more time i will furnish the community with a more extensive report on my findings however i'm sure if you take the time and research the subject you will be enlightened.
I have searched. There are not "thousands" (learn to count) and what there are have no consistency of model, parts, or age of phone. There are ALWAYS an incidence of loose connections in any device. That's why there are warranties. There is NO specific issue with speakers.
Considering the 200 million iPhones out there, the incidence of cold solder connections is very small. Check out the reports for other devices for a meaningful comparison. While your at it, sign up for courses in statistics and analytical thinking.