In short, there are no documents from this period. Everthing to do with pronunciation is founded on reconstructions supported by a number of theories, which are based on some assumptions, that more often than not, reflect the recent or modern geopolitical leanings, of their academic progenitors and advocates alike. The earliest surviving text in Alemannic is the Bülach fibula; a very short runic script, which dates between the 3rd and 6th centuries AD. The earliest known East Germanic text is the Codex Argenteus (6th century AD) which includes Ulfilas's 4th century AD translation of the Greek Bible into Gothic.
I hope this may help.
CmacQ
Bookmarks