How likely is it that fever and a heart murmur is linked to toothache and abscess formation?

Yes and no: An abscess is an infection which can definitely have a fever associated with it, but it can not cause a heart murmur.

Endocarditis: If this is a new murmur and fever in the setting of anorak infection you need to be concerned about endocarditis which is an infection of a heart valve which can be very concerning. I would recommend that you see your physician asap.