You can't really increase the sound of the Ecoboost engine itself, and the sound you do hear is 'piped' in artificially through the audio speakers anyway.
(Even the GT's pipe the sound into the cabin, but use a sound tube from the intake to the firewall).
You can however increase;
1. The exhaust note - By fitting a catback, turboback or axle-back aftermarket exhaust system.
2. Increase the air intake 'sucking' sound - By fitting a Cold Air Intake like a Roush, JLT, Airaid, etc.
3. Vent the Blow off Valve (BOV) to atmosphere for the 'peeeesh' sound as you come off boost - Either by disconnecting the outlet rubber hose on the BOV and capping the hose inlet on the intake, or installing a vacuumed operated aftermarket BOV, such as a Turbosmart one. (The stock BOV is electronic).
You could get a little more engine sound in the cabin, but only slightly, by removing the insulated noise absorbing pad under the hood.