a preschooler with autism who is always singing or babbling

[ZenaAhmed]

My son is to be four in one month. He is always singing nursery rhymes and cartoon character tv songs
that he wont listen when u we are trying to talk to him or teach him. how can we stop him from doing
that and listening and concentrating on what we are telling him

[Sally44]

Try googling echolalia and delayed echolalia in autism to see if that is relevent.  Alot of children on the spectrum repeat things they have heard.  It might be an immediate repetition of something you ask for say to your child eg. 'do you want a drink' might be repeated back to you by your child 'do you want a drink'.  But it can also be delayed repetition of things heard on TV/DVD/conversations at home or school/adverts/any kind of noise.
My son is verbal, but also uses alot of echolalia phrases put together like puzzle pieces to make up his speech.  He also uses the same accent as it is initially heard and although we are in the UK he speaks with an American accent.
An example of delayed echolalia is this: a teenage boy in class was asked by the teacher to concentrate and complete his work, to which he replied 'go to hell lieutenant'.  He was reprimanded for this, but when the teacher spoke with his parents she discovered that he had been watching a war film in which a soldier was told to do something he didn't want to do by his commander and that was the response the soldier gave 'go to hell lieutenant'.  What does this tell us?  That the child understood the emotional context of the phrase and used it in a similar situation.  Unfortunately the teacher had no understanding of the relevance of this phrase (or the fact that it had come from a film) and thought the child was being either silly or rude.  Infact he was being neither.  He was simply communicating using a phrase he had learnt that he thought was relevant to the situation.
For those whose communication difficulties are more severe they may use echolalic phrases, adverts, songs, jingles etc to 'take their turn' in conversation.  So you may ask your child a question.  They may not understand what you have said, or if they do they may not be able to formulate a response.  But they do understand that it is their turn to speak and so they repeat an echolalic phrase.  For example I asked my son 'was school good today', and he replied 'there's a hider in the house'.  I know that is a phrase from a children's TV programme he watches.  On other occasions he is perfectly okay to answer my questions using his own words.  
Some children/adults with autism find eye contact uncomfortable or even painful.  Others cannot look at you and also listen at the same time.  To get their attention repeat their name until they acknowledge that they have heard you then tell them what you want to say.  You don't need to make eye contact to convey the message and in many cases deliberatly not seeking eye contact and even turning your back on them or speaking to the wall means that they are able to hear what you are saying better because they don't have the additional visual distraction of having to look at you.
But, having said that, some children/adults are okay with eye contact, so you just have to see what works best.

[iamsympatico]

I find that asking our child to look at you well you are speaking to him/her helps to get their attention. Do not start speaking until they are looking at you and get down to their level

[Jason C Bourret, Ph.D., BCBA-D]

Delayed echolalia and vocal stereotypy are not uncommon in children diagnosed with autism and, as you are experiencing, can pose a challenge in that they can interfere with learning.  This type of behavior, although in some cases communicative, is often a form of self stimulation.  As opposed to problem behavior that serves a communicative function, stereotypy of this type can be quite challenging to treat.  With communicative behavior, one of the most effective treatments is to teach the child an appropriate alternative method of gaining whatever is being sought through the problem behavior and not allow problem behavior to result in this same consequence.  Thus, the appropriate behavior works, the problem behavior doesn’t and the child learns to do what works.  In the case of vocal stereotypy and delayed echolalia, the stereotypy always works to produce the sound.  That being said, there are interventions that have been shown to reduce the likelihood of stereotypy.  They may involve the use of competing sources of stimulation (e.g., music) either noncontingently (in cases in which the child does not emit the stereotypy when preferred music is playing, for example) or contingent upon the child spending a certain amount of time without engaging in stereotypy.  Other common treatment procedures make use of overcorrection, thus prompting the child to say a few words each time he or she engages in stereotypy.  This is designed to make it a bit of an annoyance for the child to engage in stereotypy and also give the child some positive practice engaging in appropriate speech.  These sorts of procedures can be effective but should be overseen by a trained behavior analyst who has experience in treating this sort of behavior.