In most disciplines and in the larger more developed countries it's perfectly straightforward to provide real-time transcription where the speech is transcribed into text on a big screen (or streamed to a delegate's laptop or tablet).
Many deaf and hearing-impaired people do not have sign language, or
fluent sign language, and as with translation between any two
languages, nuance can be lost and ambiguities can be introduced in the
translation from English to Sign. Sign interpretation also requires
that a student maintain continuous eye contact, usually necessitating
additional notetakers. Students using real-time transcription can take
their own notes; the last dozen or so lines of the lecture are
displayed on the transcription screen for easy reference.
(adapted from Mirabai Knight Stenography)
Using computer-assisted realtime transcription (CART) is useful not just for people with hearing impairment; people whose first language is not the same as the speaker's, or people with some specific learning difficulties, may find it useful to have a real-time transcript.
In the USA, one in seven people have hearing loss. For people over 65,
that rate goes up to one in three. Events at most conferences seat
hundreds of people, so statistically it’s a sure bet that at least
some of those people would benefit from captioning. Even people with
mild hearing loss, who do quite well in one-on-one social situations
by using a combination of residual hearing, lip reading, and context
clues, often have trouble with conference audio, which can be
distorted in the amplification process, and which puts the speaker so
far away from the audience that lipreading becomes impossible. There’s
also the benefits that captioning can offer people without hearing
loss, who may be more comfortable reading written English than
understanding spoken English (very common when English isn’t a
person’s first language), or who may have central audio processing
issues (very common in Aspergers and autism) or attention deficit
issues such as ADHD.
And the transcript forms a useful record of the conference, and can be used by people who couldn't attend the conference because of cost or distance, or who have disabilities making travel impossible or difficult, or who are unable to obtain visas. The transcript can also be translated (or machine-translated, which may be good enough for getting the gist or deciding whether ot order a paid-for translation).
All this widens access to and participation in the conference, and means that the cost of providing the transcript can be covered by the general budget rather than being a specific cost for one or two delegates.
An example of a conference video being transcribed
And a picture of a conference with, it appears, both transcription and sign.