Learn Icelandic: Við tölum mikið saman.

This happens because of a very common Icelandic vowel change called the 'u-shift'. When an a in the main part of a word is followed by an ending that contains a u (like the -um ending for 'we'), the a changes to an ö. So, tala + um becomes tölum.

Do all Group 1 verbs change their vowel in the 'we' form like this?

Only the ones that have an a in the main part of the word! The -um ending causes this specific shift from a to ö. If a Group 1 verb has a different vowel, like skoða (to look at), it simply takes the ending without changing the inner vowel, becoming við skoðum.

Why is mikið used here for 'a lot'?

Here, mikið is acting as an adverb describing how much the action (talking) happens. It is actually the neuter form of the adjective mikill (much/great), which is the standard way to turn it into an adverb meaning 'a lot' or 'much' for verbs.

Is tala saman a special phrase?