With the CU7 release Microsoft has added Music on Hold abilities to the Lync Phone Edition clients. Many organizations are starting to turn this feature on now, but it's possible that external PSTN callers still might not not hear any music on hold being played. If you're seeing (or not hearing) this behavior, there may be a gateway setting that's not passing the music on hold to PSTN callers.
On an AudioCodes MSBG 1000 this setting can be found under VOIP\GW and IP to IP\DTMF and Supplementary Services\Supplementary Services. Change the Hold Format parameter to "Send Only" if it is currently set to 0.0.0.0. This parameter controls if the gateway should expect a SDP with fields set as a=sendonly and c= containing the client's IP address, or if it should expect a=inactive and c=0.0.0.0. Setting this value to Send Only allows the Lync PCs and phones to successfully pass music on hold to a PSTN caller.