Post the first third of your dxdiag here. The whole thing won't fit in a single post nor is it all needed. It looks like a resource issue. As you add more enemies, friendlys or sounds your computer is stumbling to keep up. 
Approximately the first 1/3 of the txt file:
perating System: Windows 7 Home Premium 64-bit (6.1, Build 7600) (7600.win7_gdr.120830-0334)
           Language: English (Regional Setting: English)
System Manufacturer: Hewlett-Packard
       System Model: p6537c
               BIOS: BIOS Date: 04/22/10 21:35:46 Ver: 6.04
          Processor: AMD Phenom(tm) II X4 830 Processor (4 CPUs), ~2.8GHz
             Memory: 6144MB RAM
Available OS Memory: 5888MB RAM
          Page File: 1642MB used, 10130MB available
        Windows Dir: C:\Windows
    DirectX Version: DirectX 11
DX Setup Parameters: Not found
   User DPI Setting: 144 DPI (150 percent)
 System DPI Setting: 96 DPI (100 percent)
    DWM DPI Scaling: Enabled
     DxDiag Version: 6.01.7600.16385 32bit Unicode
------------
DxDiag Notes
------------
      Display Tab 1: No problems found.
        Sound Tab 1: No problems found.
        Sound Tab 2: No problems found.
          Input Tab: No problems found.
--------------------
DirectX Debug Levels
--------------------
Direct3D:    0/4 (retail)
DirectDraw:  0/4 (retail)
DirectInput: 0/5 (retail)
DirectMusic: 0/5 (retail)
DirectPlay:  0/9 (retail)
DirectSound: 0/5 (retail)
DirectShow:  0/6 (retail)
---------------
Display Devices
---------------
          Card name: ATI Radeon 3000 Graphics 
       Manufacturer: ATI Technologies Inc.
          Chip type: ATI display adapter (0x9616)
           DAC type: Internal DAC(400MHz)
         Device Key: Enum\PCI\VEN_1002&DEV_9616&SUBSYS_2AA9103C&REV_00
     Display Memory: 2940 MB
   Dedicated Memory: 252 MB
      Shared Memory: 2687 MB
       Current Mode: 1920 x 1080 (32 bit) (59Hz)
       Monitor Name: HP 2310 Series Wide LCD Monitor
      Monitor Model: HP 2310
         Monitor Id: HWP288E
        Native Mode: 1920 x 1080(p) (60.000Hz)
        Output Type: DVI
        Driver Name: aticfx64.dll,aticfx64.dll,aticfx32,aticfx32,atiumd64.dll,atidxx64.dll,atiumdag,atidxx32,atiumdva,atiumd6a.cap,atitmm64.dll
Driver File Version: 8.17.0010.0024 (English)
     Driver Version: 8.733.0.0
        DDI Version: 10
       Driver Model: WDDM 1.1
  Driver Attributes: Final Retail
   Driver Date/Size: 5/12/2010 02:19:40, 584704 bytes
        WHQL Logo'd: Yes
    WHQL Date Stamp: 
  Device Identifier: {D7B71EE2-D556-11CF-9473-A30ABEC2C535}
          Vendor ID: 0x1002
          Device ID: 0x9616
          SubSys ID: 0x2AA9103C
        Revision ID: 0x0000
 Driver Strong Name: oem61.inf:ATI.Mfg.NTamd64.6.1:ati2mtag_RS780:8.733.0.0:pci\ven_1002&dev_9616&subsys_2aa9103c
     Rank Of Driver: 00E60001
Is that enough of dxdiag?Thanks,
Rot