Should you choose to insert any sort of audio or video content, you'll see a small "Play" button that lets you preview it to ensure it's what you want.
You can also rearrange your multimedia content after dropping it into a message, though the process can be a bit awkward at first, since the controls aren't exactly intuitive. For example, you need to scroll over and highlight the item you wish to move, then click your trackball/trackpad and choose "Move Field." From there, you can scroll your cursor up or down in the message to shift your content into place and click the trackball again to confirm your movements.
Simply click the Menu key again when finished, and send the message like you would any other BlackBerry correspondence. Because MMS messages often contain a lot of data, it can take a few minutes for your message to process. So be patient.
When your recipient receives your MMS slide show and opens it, your content starts "playing" immediately, though Mute and Volume options let them quickly modify audio if the need be. In fact, it's a good practice to let MMS slide show recipients know in the subject line that your message contains audio.
(Note: You can modify your MMS settings, via your BlackBerry Options, typically a wrench-like icon on your home screen, and MMS options item to block MMS messages and/or customise more specific options. If you're having issues receiving MMS messages, check these options and make sure your multimedia reception option is set to "Always." )