AdPlug claims to support .adl files (Westwood AdLib-format), but the plugin crashes on them. I just tried the DOS-port to verify. Allthough AdPlay loads .adl files it doesn't produce any sound output. Hence .adl support is broken in the current release, perhaps it never really worked at all.
There is just one application known to me that can play .adl files, named RdosPlay. As the name suggests, its a DOS application and requires OPL2 support. Surprisingly it refuses to work in DOSBox, but I was able to run it in a DOS-Console from Windows using the VDMSound driver.
RdosPlay v0.21b can be obtained from MODLAND: download
VDMSound v2.1.0 is available at VOGONS: download