Boo, so I decided to do some research into why Classic Minecraft was not having any sound.

1. From the website, Classic no longer downloads the /resources folder.
2. A faulty c.class file which is not playing back music if places in a newly made /resources folder.

So from this I present to you how to fix this!

Tutorial Video: http://www.youtube.com/watch?v=tqUmQWfmlV8

Download https://github.com/downloads/zhuowei/zhuowei.github.com/mcsndfix.zip and extract. You will find the fixed c.class file inside. Open up your Minecraft.jar in the /lib directory and navigate com/mojang/minecraft and put the c.class file there, overriding the old file. Zip the entine Minecraft folder back up and rename to minecraft.jar and override the old .jar. We now have a fixed minecraft.jar ready to play music!

Question is, where do we find this music?

If you already own Minecraft then you can create a new folder in /lib called "resources". Then put the following tracks from the resources folder in .minecraft into the new resources folder:

stone3.ogg
stone1.ogg
grass4.ogg
wood3.ogg
wood1.ogg
wood4.ogg
grass1.ogg
stone4.ogg
stone2.ogg
grass3.ogg
gravel1.ogg
gravel2.ogg
gravel3.ogg
gravel4.ogg
grass2.ogg
wood2.ogg
calm3.ogg
calm1.ogg
calm2.ogg

If you do not have premium Minecraft then you can download the sound files from here: http://www.mediafire.com/?6ziwnqjipl0981b then extract the files into the resources folder that you created in the /lib directory.

If there is a /minecraft directory in your client then delete it so the new minecraft.jar can take effect.

Then simply play on your favourite server (WoM:Realms) and hear that lovely sound (make sure it's turned on). I have seen that it may take a little longer to connect but it's woth it!

If you have any troubles then write in the comments below and I will assist you as this can be quite confusing :)

Source: http://www.minecraftforum.net/topic/647308-sound-broken-in-classic/ Major thanks to the guy who fixed the c.class file :D