Note: MARKETDELTA PROFESSIONAL IS REQUIRED FOR CUSTOM AUDIO ALERTS

Attached to this article is a chart definition named BigTradeMonitor.txt which you could import and use if you have the Pro version.  

To import the attached chart definition, save it to your computer and then in MD click File | Import | Chart Definition.  It will open a chart with the definition applied.

To change the sound double-click in the top pane and then double click on the SIGNAL to bring up signal pref then change the "Signal Action" towards the bottom to a different sound.

To change the volume level (setup to >= 200 by default) in that same signal marker window and click the "Edit" button at top next to signal name and in the signal window and change VO >= 200 to whatever level you want.  You could make that VO = 200 if you ONLY wanted to hear 200 size trades.

Here is a short video that provides an overview with for working with audio alerts: