Unfortunately I can't write a plugin for each and every browser out there! But, there is a solution for you ;)
Go ahead and install the "Greasemonkey" extension for your firefox browser. Great, now that you have the extension installed, you'll need to add a new script. You can click here to install the script for SCD.
First of all, you will need the "Greasemonkey port" extension for SeaMonkey. You can Download it here. After the installation has completed, you'll need to add a new "script" to Greasemonkey. You can do that, by simply clicking this link: click here to install the script for SCD.
All the other browsers
You will have to see if there is any "Greasemonkey" port for your browser ()it might be called differently though). Once installed, you can click the following link to install the script: click here to install.
How to know if the script is running?
You'll see a new button appear under every song on soundcloud.com.
When using the browser extension with SCD 2.7.0 or above, you have an option which will automatically quit SCD when the download completes. You can read more about this here...