1. Via the Audience tab
2. Via the Access tab
If a Custom Role does not have the permission to run the underlying report (eg. Sales Report) but has been added as part of the Audience, the report will not be viewable since technically, the user does not have the permission to run it. This means, the user must be added from the Access tab of the said report to be able to access the report.
On the other hand, if the Role has been given the Permission: Reports > Sales. Then, the User is able to view the Report as long the user has been added as part of the Audience. This is because the user has an access to the underlying report.