Here's where you'll find information on Redwall and medieval songs, as well as samples of those songs; classical music; Celtic music; and so on and so forth.  Everything except the Songs of Redwall section is made up of links to other sites.

Songs of Redwall - An archive of Redwall songs, poems, and riddles

bullet Mossflower - Compiled by Rowan Treeleaper
bullet Mariel of Redwall - Compiled by Rowan Treeleaper
bullet Mattimeo - Compiled by Rowan Treeleaper
bullet The Bellmaker - Compiled by Rowan Treeleaper
bullet Pearls of Lutra - Compiled by Rowan Treeleaper
bullet Redwall - Compiled by Griever
bullet Salamandastron - Compiled by Griever
bullet Martin the Warrior - Compiled by Griever
bullet Outcast of Redwall - Compiled by Snowspine
bullet Marlfox - Compiled by Snowspine

Medieval Music

bullet The Contemplator's Folk Music Site - An excellent resource.  This site contains folk music from England, Scotland, Wales, Ireland, and America.  They have tunes, information, lyrics, historical background, and links.  Probably the best resource for medieval music that I've found yet.
bullet Medieval Sheet Music - Free sheet music from medieval-period compositions.
bullet Cantaria - Celtic folk songs.
bullet SCA Songs - A collection of medieval filks.  According to the Filk Song Forum, a filk is "a song with original lyrics written to an already established melody for an old or traditional song," and is usually "humorous or genre-related."  Why not just call them parodies?  I have no idea.
bullet The Scribe Database - A "systematic collection of scores, color images, texts and bibliographic information of medieval music which can be searched by text or melody and which will return musical information in the form of a modern score, text data and, where available, a color facsimile of an original manuscript. In contains a complete annual cycle of liturgical chant taken from original medieval sources and complete works of selected composers from the twelfth to the fifteenth centuries." You can look up the big words like "liturgical" and "facsimile" on your own.  I don't know what they mean either.
bullet Dagonell's Bardic Notebook - A collection of Irish traditionals, filks, and other medieval songs.
bullet The Black Book of Locksley - Medieval folk songs and filks.  Also includes a few plays.
bullet Ancient Music of Ireland - Irish lyrics and music.