I think there is still an issue with this loop. If you remove an element from the list, you shouldn't increase the index, otherwise you would skip an element.

    public void Update()
    {
        int index = 0;
        while (index < _activeSoundEffectInstances.Count)
        {
            SoundEffectInstance instance = _activeSoundEffectInstances[index];

            if (instance.State == SoundState.Stopped && !instance.IsDisposed)
            {
                instance.Dispose();
                _activeSoundEffectInstances.RemoveAt(index);
            }
            else
                index++;
        }
    }

Alternatively, you can iterate backward to simplify this and have a predictable loop:

    public void Update()
    {
        int index = 0;
        for (int i = _activeSoundEffectInstances.Count - 1; i >= 0; i--)
        {
            SoundEffectInstance instance = _activeSoundEffectInstances[i];

            if (instance.State == SoundState.Stopped && !instance.IsDisposed)
            {
                instance.Dispose();
                _activeSoundEffectInstances.RemoveAt(i);
            }
        }
    }

I think backward iteration is a better solution.
Gemini suggests an improvement to this code.
I think this version is a little long, but more appropriate.
Can I commit this version?

public void Update()
{
    for (int i = _activeSoundEffectInstances.Count - 1; i >= 0; i--)
    {
        SoundEffectInstance instance = _activeSoundEffectInstances[i];

        if (instance.State == SoundState.Stopped)
        {
            if (!instance.IsDisposed)
            {
                instance.Dispose();
            }
            _activeSoundEffectInstances.RemoveAt(i);
        }
    }
}

I think backward iteration is a better solution. Gemini suggests an improvement to this code. I think this version is a little long, but more appropriate. Can I commit this version?

public void Update()
{
    for (int i = _activeSoundEffectInstances.Count - 1; i >= 0; i--)
    {
        SoundEffectInstance instance = _activeSoundEffectInstances[i];

        if (instance.State == SoundState.Stopped)
        {
            if (!instance.IsDisposed)
            {
                instance.Dispose();
            }
            _activeSoundEffectInstances.RemoveAt(i);
        }
    }
}

Yes, this version would be more preferable, please update @nahaharo