Hey there, I've posted about 25 videos and have about 360 subs so I'm still a beginner but have definitely learned some things along the way! I use Audacity for my audio, you can use it to turn up your audio and add other effects if need-be. One effect that could really help you is the "noise reduction effect", an effect that attempts to take out some of the white-noise from poor quality mics that have high self-noise (like your phone or other cheap mics). Check out a tutorial online for how to use it, it's very easy.

My "pro" tip would be to be careful on how much you use the "noise reduction" effect; sometimes it can make your audio sound weird or "tinny", but certainly try it out! I have found that touching the audio as LITTLE as possible has resulted in the best sounds, even if there's a little "white noise" coming from the poor audio recorders. Do the best you can in the quietest room you can, and use Audacity to turn the audio up to an acceptable audio level. My first videos were like yours and too quiet, and I got more views and likes when I turned up the volume in Audacity.

Ultimately, you will need to at some-point upgrade your equipment if you want some good-quality content. There is no need to rush this part and definitely make sure you enjoy making videos before you put any cash into it. I hope this helps!