Here's why we don't know:
* No Rosetta Stone: We haven't found any bilingual inscriptions that could help us decipher the script.
* Limited Evidence: We have thousands of Indus Valley seals and inscriptions, but they contain a limited number of symbols.
* Unknown Language: We don't know which language the script represents, making it difficult to analyze the symbols and their potential sounds.
While we can't definitively say if the Indus Valley script had a sound like "a", it's possible that it did. It's also possible that it used a different system altogether, one that didn't include individual letters.
Until the script is deciphered, we won't be able to say for certain if it had an "a" or any other specific sound.