I was doing some research for a friend about how to sell music on-line. Here is what I found.
There are some considerations you will need to address with your delivery of web services for your music clients that want you to produce a web site for them to sell music via download.
Here are the major music distribution services pages related to getting music and related media distributed on them. You will need to set up accounts on here for your clients and for your self.
Apple - iTunes - Working with iTunes - Sell Your Content - Music Provider: FAQs
Google Play for Artists: Sell your original music
Amazon Seller Services
Once you have an account on each of these then I can help you set up your client's site to sell music downloads.
If you wish to accept and process payments on your clients web pages then you will need to also activate a SSL certificate with GeekZoneBooks.Com ( Digital Certificate
http://waptugdomains.supersite2.myorderbox.com/digital-ssl-certificate/index.php ) for the site so you can securely accept credit card info.
Regarding the CGI contact form....
PHP mail() Function
If you need me to spend my time to create forms and such for you clients....
I charge $65/hr It takes me about 2 hours to set up a form system.
I accept payment via paypal in advance for development services.