playSound is a static method meaning it exists when the program is loaded. audioSounds and minTime are SoundManager instance variable, meaning they will exist within an instance of SoundManager. You have not created an instance of SoundManager so audioSounds doesn't exist (or it does but you do not have a reference to a SoundManager object to see that).
To solve your problem you can either make audioSounds static:
public static List<AudioSource> audioSounds = new List<AudioSource>();
public static double minTime = 0.5;
so they will be created and may be referenced in the same way that PlaySound will be. Alternatively you can create an instance of SoundManager from within your method:
SoundManager soundManager = new SoundManager();
foreach (AudioSource sound in soundManager.audioSounds) // Loop through List with foreach
{
if (sourceSound.name != sound.name && sound.time <= soundManager.minTime)
{
playsound = true;
}
}