Rule-based presentation of related content items

Features are disclosed for presenting multiple content items based on one or more rules defining how the multiple content items are to be presented. One content item may be a primary content item, and any number of additional content items may be related to or supplement the primary content item in some way. Rules may define which related content items may be presented and, in the case of multimedia content (e.g., content with both audio and video elements), which elements of the primary content may be overridden by or mixed with related content, and which may not. References to related content may be embedded in the primary content item, or references may be accessed at a content management system or service.