I make my backing tracks with a virtual orchestra. The music is sequenced with Cubase. All instruments are from the Vienna Symphonic Library (VSL) and are rendered with the Vienna Instruments Pro plugin. The orchestra is placed in a virtual version of the great hall in the Vienna Konzerthaus.
For Carmen I have set up a template with the instruments positioned as shown in the image below. The 1st violins and violas are seated to the left and the cellos and 2nd violins to the right. The double basses are in the center with the woodwinds behind them: 2 flutes, 2 oboes and 2 bassoons. The 2 clarinets are seated behind the oboes. In the back we have on the left the harp and 4 horns, and on the right 2 trumpets and 3 trombones. In the far back we have the percussion with a snare drum, cymbals, tambourine, timpani, bass drum and triangle.
The castanets are not shown in the picture because I positioned them on stage, where Carmen is supposed to play them. Also not shown are the instruments that are doubled: The 2 piccolos are in the same position as the flutes and the cor anglais is in the same position as the 2nd oboe. Finally, the on- and offstage cornets are also not shown. I create the offstage effect by making the sound very wet and soft.
For La Traviata I use the exact same template with the addition of a Cimbasso to the trombone section.