To add to the above. Before extracting the audio stream it is possible to trim the recordings with nicesplice. With radio recordings, bookmarking with the remote control handset does not work, but you can add bookmarks through Webif. What I do is play the recording over the network on an Android tablet (ES file explorer [LAN] and MX player), you can do similar with a PC (Windows Explorer [Network] and VLC or Windows Media Player) to get the start and end times (to the second) and then I bookmark these times through Webif. If you then trim (nicesplice) and extract/ convert the audio track, the resulting mp3 file contains just the portion of the audio you want.
Note that if you trim a radio recording with nicesplice (through Webif) the trimmed ts file will be unplayable on the HDR-FOX itself, but the file extraction/ conversion will still work. If you want the ts file to play on the HDR-FOX, run nicesplice from the command line (Telnet session) with the '-noStripEPG' option. How to do this is described in the wiki.