First of all, congratulations! Maybe you could make a video to thank your subscribers and let them know directly that they are appreciated. Watching other people's subscriber milestone videos may give you some ideas on that. If 30 subs has convinced you of a new goal (such as posting more often, upgrading equipment, or anything you think your subscribes may be interested in knowing) mention that in the video too
If you'd rather not dedicate an entire video to it, you could just briefly mention it at the start or end card of your next video.
From watching one video I can say that your audio quality is really good! Keep it up!