Connect your input source to your laptop. If your laptop contains a 3.5mm "Line-In" jack, you can connect tape decks, turntables and instruments using 3.5mm adapters. For example, if connecting a guitar, use a 3.5mm instrument adapter -- 1/4 inch to 1/8 inch -- on the end of your guitar cable. If connecting a tape deck or turntable with RCA -- red and white -- jacks, connect an RCA-to-3.5mm adapter, or just use an RCA-to-3.5mm cable. If your laptop contains only USB, you can purchase specialized USB adapters for use with instruments and other inputs, available at music supply stores.
Open a digital audio program for your audio output. If you do not have any programs for external audio playback, download a free program, such as Audacity, Sound Engine or Sweep. Many other programs are also available for free.
Open the "Options" or "Preferences" in your new software. You should find it on one of the menus on the menu bar along the top of your screen, such as the "Help," "Tools" or "Options" menu. When your options menu appears, select the "Audio Options" or "Audio Output" menu, if asked to select a submenu or tab.
Select your output source from the list of available outputs. Depending on your operating system, your default sound card driver should read "Core Audio" (Mac) or "ASIO" (Windows), either of which will allow you to play audio through your laptop speakers. If you do not see either of these options, look for a "Line," "Line Out" or "Built-in Audio" option. Make your selection, and exit your "Options" menu.
Begin transmitting music. If using a tape deck or other playback device, press "Play." If connecting a musical instrument, begin playing. Use the "Volume" or "Master Volume" slider on your software window to set your output volume, and use your "Effects" or "EQ" options to alter the sound of your input.