Is engine sync to avoid beat frequencies also a concern for jet engines?
Some aircraft with turbofans have an engine synchronization system for exactly that reason: reducing the beat resulting from different, but similar, frequencies for the two engines.


MD-80: The McDonnell Douglas MD-80 can synchronize its engines by EPR, N1 or N2 when the autothrottle is engaged:


  Engine Synchronizer System
  
  The engine synchronizer system automatically matches the N1 or N2 speed or
  EPR of left engine to match right engine. The system includes a three
  position (N1, OFF, N2) ENG SYNC selector, ENG SYNC ON annunciation, and control
  system with actuator to trim left engine fuel control. The pilot can override
  the system at any time with throttles.
  
  When the ENG SYNC selector is placed in N1 or N2, signals from the tachometer
  generators causes engine synchronizer system to trim left engine fuel control
  until left engine RPM (N1 or N2) matches right engine.


(MD-80 FCOM Sec. 18 Power Plant - General)
CRJ: The Bombardier CRJ-700/-900 can also synchronize by either N1 or N2 via the FADEC:


  N1 and N2 Synchronization
  
  In the cruise range, thrust is set to a specific value by the FADEC based on throttle
  position. Synchronization allows the FADEC to match the fan (N1) or the core (N2)
  speed of the two engines for noise reduction. Synchronization is selected by using the
  SYNCH switch on the engine control panel . The left engine is designated as the
  master engine and the right engine will be slaved to it.
  
  For N1 synchronization to be enabled, the right engine N1 must be within 1.5% of the
  left engine N1.
  
  For N2 synchronization to be enabled, the right engine N2 must be within 7.5% of the
  left engine N1.


(CRJ FCOMv1 chapter 20 Power Plant - Thrust Control)


As far as I know, no Boeing or Airbus aircraft has an engine synchronization system. Presumably, the beat is less of a problem for wing mounted engines since the passenger cabin is shielded from direct sound by the wings. For tail mounted engines, engine noise is more of a problem in the cabin.