Liason between sommes and arrivés

Yes, it is optional. 

Not surprising that with slower speech as in this dictation, optional liaisons are not made, although the required liaisons are there, of course. 

In everyday speech, optional liaisons are regularly skipped, and they are often considered to represent more formal speech. Nonetheless you will hear them used, and sometimes even the same speaker will switch between making them and not during a conversation. 

 There is a good series of articles on the Laura Lawless site. Here is the link to a discussion of optional ones : https://www.lawlessfrench.com/pronunciation/optional-liaisons/