I don't know enough about OpenACS' new way of dealing with mime types, but maybe inserting the desired type in cr_mime_types manually will help (see acs-content-repository/sql/oracle/content-create.sql). Otherwise did you try adding a line in the nsd config file:
ns_section "ns/server/${servername}/mimetypes"
ns_param .mp3 audio/the-mp3-type
Regarding storing the data in the db versus file system: you definitely want to use the content repository, which can transparently use either storage method. Check out one of the numerous posts here on that topic.