[fragnichtnach]

The result of some researches:

Joining a server and downloading new *.mp3 will not make them working immidiately. You need to restart CS:GO. Probably something like "snd_rebuildaudiocache" (what has been removed some weeks ago) would help.

Is there a way to make it possible to play custom sounds directly after the are downloaded?

Here is the current code I am using:

#include <sourcemod>
#include <emitsoundany>

#define MAX_FILE_LEN 80

Handle g_CvarSound = INVALID_HANDLE;
char g_soundsound[MAX_FILE_LEN];

public void OnPluginStart() 
{
	SoundFilespluginStart();
	AutoExecConfig(true, "testsound");
	RegConsoleCmd("sm_testsound", Command_Sound);
}

public void SoundFilespluginStart() 
{
	g_CvarSound = CreateConVar("sound", "Sound.mp3", "Sound");
}

public void OnMapStart() 
{
    precache_sounds();
}

public void precache_sounds() 
{
    GetConVarString(g_CvarSound, g_soundsound, MAX_FILE_LEN);
    char buffer[MAX_FILE_LEN];
    PrecacheSoundAny(g_soundsound, true);
    Format(buffer, sizeof(buffer), "sound/%s", g_soundsound);
    AddFileToDownloadsTable(buffer);
}

public Action Command_Sound(int client, int args) 
{
	CreateTimer(0.1,PlaySound);
	return Plugin_Handled;
}

public Action PlaySound(Handle timer)
{
	float volume=0.3;
	for(int client=1; client<MAXPLAYERS; client++)
	{
		if(IsClientInGame(client) && !IsFakeClient(client))
			EmitSoundToClientAny(client,g_soundsound,SOUND_FROM_PLAYER, SNDCHAN_AUTO, SNDLEVEL_NORMAL, SND_NOFLAGS, volume, SNDPITCH_NORMAL, -1, NULL_VECTOR, NULL_VECTOR, true,0.0);
	}
}