Below are links to resources for building audio projects.

ExpressPCB PCB house that provides free Windows schematic and printed circuit board layout software and inexpensive builds of 3 small boards for experimenters ($51 without silkscreen or soldermask, or $75 with both), plus other options
Tone Stack Calculator Free Windows program that contains many tone circuit schematics, lets you enter your own, and displays the frequency response while you twiddle the knobs
Linear Technology Software Free software including a version of Spice circuit simulation, and Excel-based tool for power supply design, an amplifier design tool, and an audio circuit noise calculator
DIY Audio Forums for audio builders
DIY Audio Projects Audio (mainly tube) projects for home stereo
Free Stomp Boxes Forums for building your own guitar effects
Build A Pedal Building and hotrodding guitar effects
Dr. Tube Guitar tube amp schematics
Ivanhoe Open source modular synthesizer design
Synthesis Technology Modular synthesizer module kits