First of all, is it that you can't speak louder without hurting your voice or straining?
If so and you are concerned about your voice, it may be a good idea to make an appointment with your doctor.
There are medical reasons for some voice challenges, but not all.
If you can make your voice louder or softer and you are struggling with knowing when to use the right voice volume in a given situation then perhaps try these tips:
You could ask for feedback from the person you're speaking to.
They may not tell you honestly, but sometimes co-workers or friends will let you know whether you're being too loud or too quiet.
If you are too quiet or mumbling, you might be able to tell by the communication partner's body language and what they say.
Do they ask you to repeat a lot or turn their body and lean forward with their ear facing you?
If you are too quiet, try raising your voice a bit and see if the other person's behavior changes.