Using the API can is it possible to upload several voicefiles and then play them in sequence to generate a single message?
In short, yes.
You can achieve this via voice scripts and tts templates. I'd recommend looking at VoiceScriptCreate and TTSObjectCreate functions in the API Manual:
https://apiforums.easylink.com/emapidocs/24/index.html
Further, the url below provides an example on using the TTSObjectCreate.
https://apiforums.easylink.com/content/creating-and-using-tts-object
Regards,Serdal