My messaging app is gone and I have hangout. How do I get messaging back? Thanks

If your camera isn't working, or if you see a message that says "No camera found": Turn off your Chromebook, then turn it back on. Try using the camera in another app, like Hangouts. If it works in that app, uninstall the app where it doesn't work, then reinstall it.

Fix hardware and system problems - Chromebook Help

First check your internet connectivity. If internet connectivity work properly then remove the hangout app and reinstall from store.

Hangouts is the default application by Google in Android for voice and video chat. You can try to uninstall it from settings > apps > all apps and if you don't find any uninstall option there, then you can disable the app and re-enable it. A guide has been provided here: http://www.pctricksguru.com/2017/02/how-to-disable-useless-google-apps-in.html

If you know how to root your phone, then you can completely uninstall it after rooting your phone.

Hangouts is Googles version of Instant Messenger or online chatting software. Google has enabled a feature that allows text messages to be routed through Hangouts so you can use the Hangouts app as your text messaging app. If you do not use Hangouts on a regular basis, there is no need to do this.

This feature is very useful for people who use Google Hangouts and wanted to integrate their text messages and hangout chats into one app.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/thomas_e8f475a8655f7b4a

Hangout is a native app from Android, it can not be remove, this is if you are using the latest Android OS.

But there is a way around, I guess you want to use another messaging app as your default sms. The first thing that you can do is to disable Hangout. You can achieve this by going to setting, apps, search for Hangout and tap it, now tap disable.

Now, I'm not sure how to enable default for a third party app, in this case a messaging app, but you can always open the new app and maybe it could prompt you to set it as default. For the sound or ringtone, there are 2 ways to do it.
1. Open the messaging app and tap the menu, from here you can set the ringtone for your sms.
2. Download and install Zedge from the PlayStore, download the ringtones that you like the most, after downloading them, the one that you want to set it as your sms ringtone, just tap set at the bottom, and choose messages or sms.

Try cleaning caches!

Gone as in where were they stored in the first place? If stored on the device memory card, they may still be there, download a file manager and search them out. If in the cloud, resync with the app like google music, sign out, sign back in via hangouts. Your question is kinda vague as we don't know where you stored the music in the first place to help undo what was done.