In this rare footage you can hear the 1116 162 play the Austrian National Anthem with it's VVVF Frequency converters. In the early days of the Siemens ES64U2, most of the locomotives had the feature of playing different melodies at each acceleration up to a certain speed.

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Credit:
Used with permission. (IG: @pic_by_spoon)

Where does the sound come from? [Verrry Detailed Explaination]
The sound is emitted by the traction motors but has it's origin in the Traction Control Unit (TCU).

The SIBAS32 TCU consists of the following modules:
- CPU (Central Processing Unit) / ZR (Zentralrechner) -
Responsible for controlling all of the high-level monitoring and protection functions of the drive system including all the cooling and wheelslip protection as well as providing a torque and flux command for the SIP (DSP) modules.
- SPU (Signal Processing Unit) / SIP (Signalprozessor) -
Responsible for controlling one PWMI (Pulse Width Modulated Inverter) / PWR (Pulswechselrichter) or 4QC (4-Quadrant Converter) / 4QS (4-Quadrantensteller) depending on its programmed software. It implements realtime signal processing and complex control algorithms on a DSP (Digital Signal Processor) as well as configurable logic devices (LCAs / FPGAs) for flexible PWM pulse generation. This is the part which is responsible for generating the 'sound' you hear whenever a train or locomotive accelerates.
- I/O (Input- Output Card) -
Used as a data interface between the CPU module and the various input and output modules. It has D/A and A/D converters on board as well as digital I/O.
- Various Input and Output modules -
Either for analogue or digital in- or out signals these cards feature signal conditioning, amplifying or converting of important realtime sensor data or control commands.
- Pulse Amplifier -
Responsible for transmitting the PWM signals as firing pulses to the gate units of each phase module. Additionally they receive the firing acknowledge signals from the gate units. In SIBAS32 TCUs this communication is done by 40mA current loops.

PWM Pulses get sent from the TCU to the gate units of each phase module. Typical phase modules have two GTOs installed.
The Gate Unit is used to control the gates of the GTOs inside the phase and chopper modules. This is the interface between the control unit and the power part of the Inverter. Each Gate Unit consists of [one] two-channel controller and [two] high-voltage units. The two-channel controller provides the signals and the energy for the high-voltage unit. Each high-voltage-unit can drive one GTO. They are locked against each other, so that only one GTO can be switched 'on' at a time. This is done by implementing logic as well as dead-time.

The high voltage PWM pulses are applied to the traction motors. Induction motors are inductive loads - therefore the current resulting from the voltage applied will be in the form of a rough sine wave. The imperfections in the sine wave have an impact on the magnetic field - creating small vibrations which get spread around the bogie and other nearby parts resulting in an audible noise.
There are many control algorithms as well as different pulse systems out there in the industry. Different types of FOC algorithms like SFC (stator flux control), ISC (indirect stator-quantities control) or DSC (direct self control) are used to generate voltage commands which then get inverse-clark transformed to get the three phase voltages. These phase voltages are then used as reference values for PWM generation. There are many methods such as:

- Carrier Based -
- Async Carrier
- Sync Carrier
- Edge Aligned / Center Aligned
- RCFM / PCFM

Optimized Sync Patterns:
- LUT - Based -
- Off-Line Optimized Pulse Patterns
- MPT (CP) / FPT (EP) / GFT (OVM)
- SHE / SHM / SCHM / CHM
----- Non - LUT -----
- On-Line Optimized Pulse Systems
- Direct Self Control

I will continue to update this video description to further clarify the source of sound on electric trains. In the meantime you can pay Beno's channel a visit and watch his (less in-depth, but easier to understand) video on this topic:    • Why do trains make interesting sounds