Sampled data files can be compressed and by doing this one can win on average 50% of the storage space. The amount of possible compression depends on the signal-to-noise ratio in the recordings. A high signal-to-noise ratio results in a better compression.
Compression of telephone speech (A-law or U-law) results in a compression of about 40%, as was experienced during the storage of the Dutch POLYPHONE material. A compression tool that is often used and is recommended here is Shorten by T. Robinson. It is available via anonymous ftp from svr-ftp.eng.cam.ac.uk.