The Copyright, Designs and Patents Act 1988 (as amended by The Copyright (Visually Impaired Persons) Act 2002) contains copyright exceptions for making copies in alternative formats for visually impaired people, under s.31A and s.31B.
S.31A and s.31B allow the making of copies of "a literary, dramatic, musical or artistic work;" or "a published edition". This may cover the content of the recording, but unfortunately, it does not extend to the making of a copy of a sound (or video) recording, and as the audio tape will contain sound recordings, section 31A and 31B do not assist.
So, if the old audio tapes are converted to mp3 format, even for access by a visually impaired person on a network, s.31A might cover the copying of the content (the lecture, recital and/or music), it will not cover the (separate) right in the recording..
The situations where accessible copies of old audio tapes can be converted into mp3 formats and put on a network include:
1) where permission has been obtained directly from the copyright owner of the content and recording included on the audio tape to create and distribute the mp3 files.
2) where permission to use the content and recording has been obtained through a licence from one of the collective licensing society. The relevant copyright licensing authority to approach in this regard would be the PRS/MCPS alliance. Their website is http://www.prsformusic.com/.
Or
3) where copyright in the content and recording included on the audio tape has elapsed - in which case, it may be freely used.