Partner site where you can create your digital voice: https://lyrebird.ai/vocal-avatar(Note that as of 8/23/18, you canNOT yet use this voice with SGD/AAC)
You record .wav files yourself; then upload to their site; they transcribe, store, and help you organize them. Then, you download messages to load into speech-generating device/app (including Tobii Dynavox products). NOTE: I recorded some messages, and uploaded them, and they were transcribed correctly - nice!
Use Message Banking app to record, manage, store messages on your PC. Then upload/download message files to speech-generating device. App is free. Runs on Windows 10 only. NOTE: I installed this app on my Surface Book 2 running Windows 10 Pro; when I launched it, the splash screen came up, but then the app died. I see in reviews on the MS App Store page for the app that some others had the same problem.