I received a message from Xbox saying I can use the text to speech feature in a party.

Hello, regarding your question I believe voice to text feature was removed from Xbox along with cortana, so the replacement now being xbox assistant. I think this was a case of cause people from games like Rocket League would use that speech-to-text feature
improperly and get away with trash talking and bullying players without leaving text message proof that they were doing it.

My name is Fernando I'm an independent consultant, I'm happy to help

Try this procedure, hope it helps. You can enable the Text Conversation Chat or Speech Conversation Chat features. To do this:- Go to the game's main menu and select the Settings tab. - Enter the Accessibility submenu. - Select Enable Text to Speech Chat or Enable Speech to Text Chat. To mute or unmute the microphone, highlight your Gamertag and press the OPTIONS button. In-game chat is enabled. You cannot hear party audio and the party cannot hear your voice. To switch to party audio, select [Party Settings] > [Chat Audio].

Hope I helped and see you soon

You can not party chat with 360 users. You can send them text messages, but not voice messages. You also can not open voice messages received from a 360 user on the Xbox One.

Sorry to say, but you cannot find them as they really do not exist. Unfortunately.

But Microsoft is collecting suggestions and feedback from the users at
http://xbox.uservoice.com

So there is already an item and a voting for the voice messages at
xbox.uservoice.com/.../5997802-add-audio-message-and-not-only-text-message

and also one for the speech-to-text issue at
xbox.uservoice.com/.../6005494-kinect-for-text-input-voice-to-text

Hi Reni! My name is Ricardo an independent advisor ready to help you with this situation!

For them to be able to hear your message, they have to activate the tech to speech feature on their side, as on your side you can only activate it to convert your text to speech or the other way around but not on their side