The Dominican Symphony has a website here
Fundacion Sinfonia , it's in English and Spanish, but it doesn't look like they have anything on the schedule in the near future, with the exception of a benefit concert on March 17 and another concert on April 2nd, and May 21.
There are always lots of concerts going on, but it's best to either check the local paper when you get here, but also check out Ticket Express for a calendar of the more pop music type concerts
Ticket Express - Sin filas, ni perder tiempo