When you refresh your SFDC sandbox you will get a new OrgID so the sync will stop working between your Marketo Sandbox and your SFDC Sandbox.
These are the steps you need to take:
1. Stop the sync in your Marketo Sandbox (admin - salesforce - disable sync button)
2. Wait 1-2 hours (in order to make sure any current sync cycle completes)
3. Refresh your SFDC Sandbox
4. Open a Marketo Support case asking support to refresh the OrgId in your sandbox
5. Once they tell you it's done, go fill in your new credentials in Marketo: Admin - Salesforce- Credentials
If you do the above it should all go smoothly. If you have any further concerns Marketo Support will gladly assist you