The library is written in C language and compiled with C30 compiler for the dsPIC family.

Build a dsPIC Oscilloscope and Spectrum Analyzer with 128x64 LCD display

Audio playback using the dsPIC on a simple breadboard layout with a free code library for SD card access from Microchip.

This project is an attempt to build a portable electronic stethoscope that selectively
amplifies body sound signals for clear auscultation by applying various digital filters
using Microchip dsPIC capabilities along with Microchip’s low noise op-amps and PGA.

dsPIC-based voice MIDI synthesizer by Michael Herman

Scott Coppersmith presents laser show controller based on dsPIC

If you are thinking to move on DSP, maybe dsPIC microcontrollers can be a good start as they have DSP engine built in that allows fast multiply and divide operations with arrays.

A great thing: software-based MP3 decoder for dsPICs, published under GPL.