You can use the "Record using my Phone" feature while adding audio prompts to your apps.
Once you click on it, you'll see the following prompt
Enter your number there and press "Call & Record". Once you do that, you'll get a call from your Exotel VN and you can follow the instructions in the IVR.
Once you're done recording, it'll appear on the dashboard like below:
That's it. You now have a shiny new recording for your IVR/audio prompt on the app attached to your Exotel Virtual Number!