Readers of Sound On Sound should be familiar with Mike Senior’s “Mix Rescue” column. He has posted a number of multitrack recordings in different styles at his Mixing Secrets Multitrack Library, a resource that is part of the online content to support his book, Mixing Secrets for the Small Studio
, (affiliate link).
There’s even a section for people starting out at mixing, featuring a number of smaller projects to get you going. Check it out!