That is called hamzat al-wasl, which means that in normal cases the alif is, as you were told, silent. Some styles will write the symbol above the alif, but others will just leave the alif unmarked: It is the same either way.
However, if it's at the beginning of speech, you can't just leave it silent. How it's pronounced in those cases is a bit tricky, since it's based on exactly what word it's on:
- If it's on the definite article "al" (ال), it is always pronounced as "a".
- If it's at the beginning of an indefinite noun (i.e. one that doesn't start with "al"), it's always prounced as "i".
- If it's at the beginning of a verb, it's pronunciation varies depending on the vowel on the third letter of the verb:
- If the third vowel is "a" or "i", pronounce the hamzat-al-wasl as an "i"
- If the third vowel is "u", pronounce the hamzat-al-wasl as an "u"
So, taking the two examples in your question:
- ?nzur is a verb. The third letter is the "zaal", which has an "u" vowel. Therefore, the hamzat al-wasl is pronounced as "u".
- ?ftaraahu is a verb. The third letter is the "taa", which has an "a" vowel. Therefore, the hamzat al-wasl is pronounced as "i".
Note that if you're dealing with irregular verbs, it can be difficult to determine which vowel to use since as-written they won't always show the natural vowels. There are a handful of these in the Qur'an (e.g. "?qduu" in 10:71 or "?bnuu" in 18:21 would be pronounced with an "i" instead of the "u" you might expect) but I don't think any of them would be an issue since they're not at the beginning of a sentence and the hamzat al-wasl would be silent anyway.