I've pulled together the audio development related links for Silverlight (the browser codec) and Expression (the authoring tool) and listed them below:
Audio & Video FAQs about Silverlight: Get the low down on how audio & video are supported and managed in Silverlight and Expression
Expression Encoder Preview: This fully functioning version will expire 30 days after installation...you can inquire about obtaining a 180 day extension here
Expression Encoder Preview Update: This update copies new templates that are compatible with the Microsoft Silverlight Release Candidate (RC) into your Expression Media Encoder templates folder. It does not replace the templates installed by your original Expression Media Encoder Preview installation, so you will not have to reinstall the preview or download a new version of the preview.
Microsoft Silverlight Site: Main home page for Microsoft Silverlight
Microsoft Expression Studio Site: Main home page for the Microsoft Expression authoring tool
Expression Studio Trial Version Download