Welcome to iMore! What do you mean by "text tones"? Are you saying that your Messages alerts stop sounding? I'm also confused as to how this happens over time; the alert is either there or it isn't. Please clarify.
Regardless, try resetting your device settings by going to Settings > General > Reset > Reset All Settings. If you're still having issues I recommend resetting your phone using iTunes and setting it up as a new device. Instructions for doing that are here: https://support.apple.com/en-us/HT201252