1. Spammers aren't going to pay the bandwidth costs to send out sound & video, and nobody is rushing to expand email to carry large files when a URL to a website works better.
Most spam is sent by zombies. So a spammer uploads their spammail *once* with sound and video attachments to a master zombie which seeds it to a few other master zombies in the network which then download it to the other zombies via a bittorrent like protocol. The bandwidth costs are negligible because it's all on the shoulders of the people whose computers got zombified and the recipients.
Those who can, do; those who can't, write. Those who can't write work for the Bell Labs Record.