DSi is used by many to access the internet on the go. The hardware on DSi supports video and audio functions. However, sometimes the software may not support the video and audio functions. Currently, there are no specific plug-ins that can address this problem. For audio functions there is some news that Adobe may develop a flash playerspecific for DSi. In terms of video functions there are again some rumors that a site along the lines of YouTube, called DSiTube may be developed. But there are no confirmed reports on this and these may turn out to be a hoax.
I own a DSi and use it a lot for daily internet access. One major problem I encounter is that Adobe/Java prohibits and prevents me from watching any type of video or music player. The Nintendo company is a supporter of the Opera software as the DSi browser. I have heard talks of a plug-in for DSi by Opera that allows you to watch videos but I have also heard of the DSi being too slow to watch videos. I want the truth on the subject and please only post true, researched answers. I don't have time to waste anymore and I'm sure you don't either.
P.S. I was using my DSi to post this message.
About the SD card and Flash, that are actually in debate, for one no-one really knows how to get it on there, and two, how is it going to magically transport the Flash from the SD card onto the DSi. The SD card has nothing to do with the internet browser: they are 2 different sections that never touch, so it would be highly unlikely it would work. The whole uproar about the SD card, as I see it, started in one of three ways. One maybe it was a plot to "guilt" Nintendo into making it true. Two, maybe it was an honest mistake and somebody just "forgot to carry the 1" so-to-speak. Or three, maybe people just want to stir up trouble. Whatever the case, I don't think it will work. I have given you most of the facts that I know here and if wish to learn more: there are tons of sites on the web, some will tell you exactly what I said, http://dsiflash.co.cc/, and some will say that it will work. In fact, one thread on this site (Kioskea) called "DSi Flash Player Struggle" has a lot of information and even somebody who says that his friend got it to work but will never say how, and somebody who says that you could buy SD cards with Flash already on it but never says what it is called or how to get it.
There has also been talk that Adobe might come out with a version of Flash that is made specifically for the DSi, however this has never been verified by Adobe and therefore, doubtful. There has been talks, too, about Nintendo coming out with a video player for the DSi however, Nintendo being Nintendo they have not said anything about this to confirm or deny it, so we will just have to wait. "Homebrew" software for a DSi Flash has been discussed as well, however as Adobe alone has the right to create and distribute any type of Flash player this is strictly illegal and prohibited.
Also there is a site called DSiCade, http://www.dsicade.com/main.jsp, that allows you to play certain games on it such as, Touch Mii, WAR GAMES, Defend Our Home, Wacky Tap DSi, Hangman, Sudoku, Puzzle, Phong, and DSiCalc. There is also a game called DSiPaint that is actually a link to another site called DSiPaint, http://dsipaint.com/. On this website there are various things to do such as play games; including DSiPaint, HullBreach Online Lite, and Twitter Lite (all three of these games are on different sites and each needs you to create an account with them - it is free and one account will work on all three of them, TWitter, DSOperaSDK, and HullBreach Online). Both DSiCade and DSiPaint provide chatrooms to talk with others, the DSiCade one is a game called Shout-Box where you send out various "shouts" to others to chat with them. DSiPaint has 3 different chat rooms which are a bit easier to use: they are, Chatroom Mario, Chatroom Yoshi, and Chatroom Luigi. Both work (I have tried them myself) and which one you use is a matter of preference. I use the one on DSiPaint that has the most people (it says the number of people in brackets next to the chatroom name.)
In terms of video there is a site in-the-making called DSiTube, unaffiliated with the makers of DSiCade, that, as far as I am aware, supposedly will be sort of YouTube for the DSi. However this website has not yet been functional and the makers of DSiCade have called this site a hoax; the amount of time and abundance of excuses it has used so far has lead many others to also believe that it really is a hoax. The site is currently visitable but you will not be able to see anything but a disclaimer saying that it will not modify your DSi in any way, a statement saying that it is undergoing maintenance, a message saying that "This page is currently being modified for the iDsi Browser. This site will soon not be accessible from desktop computer, portable computers or Nintendo DSi's without iDsi Browser. Please upgrade to the iDsi Browser to access this site in the future" and a little tid-bid on DSiTube saying,
"What is DSiTube?
DSiTube is a website to watch your favorite videos on the go. You can comment, rate and even subscribe to people's DSiTube Channels!
What is the point of DSiTube?
To let Nintendo DSi users stream multimedia to their device for convenience and entertainment.
Is DSiTube finished?
To be honest, no. Audio lags horrible and because only 2 videos have been properly transcoded for DSiTube it wouldn't be much of a website to use.
FAQ will be updated frequently. Any other questions you have, just email at email@example.com!"
All of that is directly for the site itself and you will notice that it referred to a "iDsi Browser"; this has currently not been released and the only word we have heard is "very soon." Also I refer you back to the site http://dsiflash.co.cc which has info on this subject as well. Will this website work? Who knows, I have given you all I know on the site, so you can now decide.
In terms of a music player I have not heard a thing, I am sorry. I have no information besides the paragraphs concerning Flash. I have given you all I know, I hope it helps.