if your sound system demands more power than your stock alternator can supply, then you will benefit by upgrading to a higher output alternator.
Assuming the sound system is a priority, what about an additional battery? I have always had 'Lightening caps' put on every sub amp, but 'lesser' (not comp level) to supplement car's audio...which is to say I understand farads (sp) and necessary level, but not what kind of pull that might have on alternator.Your friend is partially correct. Newer cars often have sophisticated electrical systems with ECU-controlled alternator output based on demand. However, this does not necessarily mean that you will need an external voltage regulator or that your dash will light up like a Christmas tree.
It is possible that the increased power draw from your additional amp and subs could cause issues with the ECU-controlled alternator output. To prevent this, you may need to upgrade your battery to a higher capacity and/or install a capacitor to help regulate voltage. It is also possible that your alternator may not be able to keep up with the demand, even with a higher output alternator.
It is always recommended to consult with a professional audio installer who can properly assess your vehicle's electrical system and recommend the appropriate upgrades for your specific needs. They may also be able to recommend solutions for integrating the additional power draw into the ECU-controlled alternator output.
Flashing the ECU may not be necessary or effective in this situation, as it is primarily responsible for controlling engine performance and emissions, not the electrical system.