Hello and welcome to Apple Support Communities anp1219,
Thank you for contacting us. This is a place where other Apple users can submit questions or respond to others to help lend a hand. Let’s take a closer look as we do our best to provide some additional insight. We understand you're having a hard time hearing your text tone intermittently. To start, make sure you have a current backup: How to back up your iPhone, iPad, and iPod touch & the most current operating system on your device. (Settings > General > Software Update) Since it's intermittent, check the Ring/Silent switch above the volume buttons next time it happens. We have also posted two resources to troubleshoot the sound issue below.
Use tones and ringtones with your iPhone, iPad, or iPod touch
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
If you need further assistance after following the link above, please don't hesitate to reach out.
Thank you for contacting Apple Support Communities.
Best regards.