A method of suggesting accompaniment tracks for synchronised rendering with a content data item includes obtaining data descriptive of the content data item. A database query based at least partly on the data descriptive of the content data item to identify auxiliary content items to a user is resolved. The database (14;28) includes records, each configured to associate descriptive data with an auxiliary content data item for forming an accompaniment track. Data representative of a selection of an auxiliary content data item is received in association with the query, and a database record for the selected auxiliary content data item is caused to be amended to reflect the selection in association with at least part of the descriptive data included in the query.