Illuminating the wall of a transparent tube

You are making this way too complicated. You don't need a opamp and a logic gate to perform your very simple logic. The on/off switch can simply be wired in series with the supply and act like a normal on/off switch. The inversion you want from the momentary button can be accomplished by a single transistor:

Q1 acts like a low side switch, with R2 providing it's base current and thereby keeping it on. When SW1 is pressed, it shorts the base to ground, which turns off Q1. The LED and buzzer are wired in parallel, not in series as you had them. It is unlikely both need the same amount of current. The LED will have about 10mA thru it when on, which lights a ordinary LED bright enough for most purposes.

Nearly the full 12V will be applied to the buzzer. You didn't say what voltage the buzzer was rated at, so I picked 12V just for example. That's a common voltage for buzzers, but they are certainly available in other voltages too. Some buzzers can appear inductive from the outside circuit point of view, so D2 protects the circuit from the inductive kickback when the buzzer is turned off. Buzzers can also make a lot of high frequency electrical noise. C1 is there to shunt most of that locally to reduce the radio interference and other bad effects from this.

The motherboard connection expect a LED.

Just use an optocoupler or an optotriac to drive next stages. I had once a 250W halogen lamp connected to the hard disk activity led via phototriac/triac :-)