The tradition of imitation as far as Eurocentrism (or the classical eras/Classical to Romantic, depending on your perspective) has usually been instrumental imitation of nature. Vivid examples include certain percussive instruments for weather, oboe reeds for babies crying, and stylistic motifs for flute compositions to evoke imagination of bird calls. The 20th century, however, has been quite different. There are examples of vocalists imitating instrumentalism. Examples? In the jazz and classical areas, there is Bobby McFerrin. He remains respected in serious circles although "Don't Worry, Be Happy" and the recording _Simple Pleasures_ on which it appeared had pop success in the late 1980s. He uses studio techniques quite a bit to create group compositions; essentially, he's singing and accompanying himself with recording technology, but his "Spontaneous Inventions" live performance is an example of solo improv. Darren "The Human Beat Box" Robinson, of the hip-hop group The Fat Boys, perfected an imitative style of a beat box, according to http://thebox.free.fr/fat.html, since Robinson's family couldn't afford a drum set (I'm assuming drum machine, actually-- the Roland SP- 12 was a standard on the early scene). Razhel "The Godfather of Noyze" of the group The Roots is the latest artist to continue this old school trend, using the freestyle method. He imitates the record scratching technique in his raps that Grandmaster Flash first refined and popularized in the very early 1980s. "Razhel vs. DJ Scribbles," with appearances by Kenny Mohammed (another beat-boxer that is touring sidekick with Razhel) and DJ Slinky is a good recording reference. See http://www.mcarecords.com/ArtistMain.asp?ArtistId=122 for more information. I can't think of other musicians per se that vocalize instrumental music at this time-- open to suggestion.1 responses total.
A number of acapella groups imitate instruments. I've heard a number of imitation trumpets, for instance. Joe Finetti of the Bobs can imitate a drum kit pretty well; unfortunately he can't do much of anything else very well. Going back to the instruments imitating vocal side, perhaps it isn't difficult to make an oboe sound like a baby crying, but I remember raising my eyebrows in surprise at the way Jorma Kaukonen could make an electric guitar sound like a baby crying.
You have several choices: