How can I fix my left earbud? It is very quiet even when volume is all the way up.

Hello,

You mean the wired headphones, correct? If so, check the charging port for lint or other debris and see this:

If your headphones don‘t work with your iPhone, iPad, or iPod touch

If no help, try this:

Go to Settings > Accessibility > Audio/Visual and scroll down to “Balance L vs R” (Adjust the audio volume balance) and make changes.

Good luck.