Ancient Hebrew, though not having written vowels, had other linguistic mechanisms that enabled the reader to discern the proper pronunciation. For example:
1) There were certain "vowel sign consonants": aleph to denote "a", he for "a", yod for "i" or "e", waw for "o" or "u".
2) Also, there were rules regarding syllable construction: The vowel of an open syllable must be long, unless accented (in which case it may be long or short). The vowel of a closed syllable must be short, unless accented (in which case it may or may not be long).
3) And, there are certain letters (e.g. he) that have a decided preference for long vowels.
4) There are rules for which syllable should ordinarily be accented.
[From The Essentials of Biblical Hebrew (Yates & Owens)]
All this being considered, the most likely pronunciation was yah-weh. But, as with all rules of language, there are always exceptions. Hebrew is like any other living language and the pronunciations and alphabets changed over the decades and centuries.
Craig