All lines that do not fall into any of the other categories are interpreted as OS/2 multimedia string commands. These lines are passed to the Media Device Manager (MDM) through the mciSendString function after the environment variables have been substituted.
Any token in the string command line bracketed by question marks (such as ?FOO?) is interpreted as an environment variable. The actual value of the environment variable is substituted into the string before it is passed to the mciSendString function. If the variable is not found, a warning is issued and the token is replaced with a NULL string. For example, assuming the environment string MMDATA is set to D:\DATA, open ?mmdata?\temp.wav alias a is equal to open d:\data\temp.wav alias a.