Hi rrecalde!
You might try a few things. Siri issues are generally always software related and can be fixed in one of the following ways:
1. Make sure that the iPhone is not connected to any external Bluetooth devices when you're trying to use Siri (i.e. headphones, wireless speaker, car stereo, etc.)
2. Toggle Siri off/on under Settings → General → Siri
3. Make sure your headphone jack is free of visible debris
4. Try utilizing Settings → General → Reset → Reset All Settings. This will not erase any content but may reset some minor settings like your background, location services preferences, saved WiFi networks, etc.
5. If all else fails, create a backup of the iPhone to either iCloud or iTunes and then restore the iPhone. After restoring, you'll want to temporarily set up the iPhone as "a new device" to see if you're still having the same issues under default settings. If the issue seems fixed at that point, attempt to bring back your backup. If the issue comes back with the backup, then the backup is corrupt and you'll need to set up as new.