The /u/ is like a French /u/ (or a German /ü/ or Danish /y/), the /a/ as in "car", and the /ó/ as in "so". The /s/ and /m/ are as in English, and the /r/ is trilled and, since it's doubled and between two vowels, lengthened like the /k/ in "bookkeeper."
Stress is always on the first syllable in Icelandic, but since this is a compound name I'm not positive whether it's SU-mar-rós or SU-mar-RÓS. I lean toward the first, as it is a compound word and I'm pretty sure it's therefore treated as one.
Plus, I like the way it sounds better *grin*.
~ Caitlín
"Blue tinted individuals who go about armed to the teeth and quoting twelfth-century poetry are not easy to comprehend." ~ Lloyd Alexander