The serving of advertisements with (e.g., on) audio documents may be improved in a number of ways. For example, a system may (a) accept information defining at least one ad spot associated with at least one instance of an audio document, (b) accept offers to have advertisements served in the ad spot(s), and (c) arbitrate among competing advertisements, using at least the offers, to determine at least one advertisement to be served in that ad spot(s). As another example, a system may (a) accept relevance information for an advertisement, (b) determine at least one audio document using the accepted relevance information, (c) present information about the audio document(s) to an advertiser associated with the advertisement, and (d) accept, from the advertiser, an offer to have its advertisement served with at least one of the audio document(s) accepted. As yet another example, a system may (a) accept relevance information for an audio document, (b) determine a plurality of advertisements relevant to the audio document using the relevance information and serving constraints of the advertisements, and (c) select at least one of the determined relevant advertisements to be served with the audio document. Examples of documents include radio programs, live or recorded musical works with lyrics, live or recorded dramatic works with dialog or a monolog, live or recorded talk shows, voice mail, segments of an audio conversation, etc.